OK, let's look at Turner, citing the RSV as the version scholars then were working with, and a French translation as its tenses tend to agree with more with the Greek than the English:
Turner 1963:62 wrote:(c) The Present which indicates the continuance of an action during the past and up to the moment of speaking is virtually the same as Perfective, the only difference being that the action is conceived as still in progress (Burton § 17). It is
frequent in the NT: Lk 2:48 13:7 (ἰδοὺ τρία ἔτη ἀφ' οὗ ἔρχεται) 15:29 (τοσαῦτα ἔτη δουλεύω σοι, and I still do), Jn 5:6 8:58 (εἰμί) 14:9 (μεθ' ὑμῶν εἴμί) 15:27 (ἐστέ), Ac 15:21 (M. γὰρ ἐκ γενεῶν ἀρχαίων κατὰ πόλιν τοὺς κηρούσσοντας αὐτὸν ἔχει, and still has) 26:31 (πράσσει, his manner of life still continues), 2 Co 12:19, 2 Ti 3:15 (οἶδας), 2 Pt 3:4,1 Jn 2:9 3:8.
Luke 2:48 (WH) καὶ ἰδόντες αὐτὸν ἐξεπλάγησαν, καὶ εἶπεν πρὸς αὐτὸν ἡ μήτηρ αὐτοῦ· Τέκνον, τί ἐποίησας ἡμῖν οὕτως; ἰδοὺ ὁ πατήρ σου καὶ ἐγὼ ὀδυνώμενοι
ζητοῦμέν σε. (RSV "... Behold, your father and
I have been looking for you anxiously."). Turner is citing the Westcott & Hort text on the authority of 01 and B, but today's editions go with the imperfect ἐζητοῦμεν. The reference time appears to be the present moment, as their seeking Jesus is just coming to an end.
Luke 13:7 Ἰδοὺ τρία ἔτη ἀφ’ οὗ
ἔρχομαι (RSV "Lo, these three years
I have come"; cf. NEG "Voilà trois ans que
je viens"). The habit of coming still holds at the present with an explicit reference time extending from the past to the present over three years. The French uses the present here just like the Greek; the English perfect is idiomatic.
Luke 15:29 τοσαῦτα ἔτη
δουλεύω σοι (RSV "these many years
I have served you"; NEG "il y a tant d’années que
je te
sers"). The activity of serving still holds at the present, with an lengthy reference time that includes the present. Note how the French uses the present.
John 5:6 τοῦτον ἰδὼν ὁ Ἰησοῦς κατακείμενον, καὶ γνοὺς ὅτι πολὺν ἤδη χρόνον
ἔχει, λέγει αὐτῷ· Θέλεις ὑγιὴς γενέσθαι; (RSV not translated; NIV "he
had been in this condition for a long time"; NEG "il
était malade depuis longtemps"). The disabled man is still disabled at this point, and the extended reference time for a long time until the present. The English pluperfect "had been" is due to an idiomatic "backshift" not applicable to Greek.
John 8:58 πρὶν Ἀβραὰμ γενέσθαι ἐγὼ
εἰμί (RSV "before Abraham was,
I am"; NEG "avant qu’Abraham fût,
je suis") is the example we've been discussing. The issue is the reference time set up by πρὶν Ἀβραὰμ γενέσθαι as it excludes the present. The French keeps the present, although it does have a perfect tense it could have used ("ai était").
John 14:9 Τοσούτῳ χρόνῳ μεθ’ ὑμῶν
εἰμι (RSV "
Have I been with you so long"; NEG "Il y a si longtemps que
je suis avec vous"). Jesus is still with them and the extended reference time is for such a (long) time. The French uses the present.
John 15:27 καὶ ὑμεῖς δὲ μαρτυρεῖτε, ὅτι ἀπ’ ἀρχῆς μετ’ ἐμοῦ
ἐστε. (RSV "...
you have been with me from the beginning"; NEG "...
vous êtes avec moi dès le commencement"). Jesus' disciples are still with him (present), and the extended reference time is from the beginning. The French uses the present.
Acts 15:21 Μωϋσῆς γὰρ ἐκ γενεῶν ἀρχαίων κατὰ πόλιν τοὺς κηρύσσοντας αὐτὸν
ἔχει ἐν ταῖς συναγωγαῖς κατὰ πᾶν σάββατον ἀναγινωσκόμενος. (RSV "For from early generations Moses
has had in every city those who preach him, ..."; NEG "Car, depuis bien des générations, Moïse
a dans chaque ville des gens qui le prêchent, ...") The (law of Moses) is still had in the cities, and the extended reference time is from the generations of the ancients.
Acts 26:31 καὶ ἀναχωρήσαντες ἐλάλουν πρὸς ἀλλήλους λέγοντες ὅτι Οὐδὲν θανάτου ἢ δεσμῶν ἄξιον τι
πράσσει ὁ ἄνθρωπος οὗτος. (RSV "... This man
is doing nothing to deserve death or imprisonment."; NEG "... Cet homme n’
a rien
fait qui mérite la mort ou la prison.") Jesus is (still) doing nothing to deserve death. Interesting, it is the French that translates with the prefect.
2 Cor 12:19 Πάλαι
δοκεῖτε ὅτι ὑμῖν ἀπολογούμεθα; ... (RSV "
Have you been thinking all along that we have been defending ourselves before you?"; NEG "
Vous vous imaginez depuis longtemps que nous nous justifions auprès de vous."). The action is still going on, and the extended reference time is "for a long time." The French uses the present.
2 Tim 3:15 καὶ ὅτι ἀπὸ βρέφους ἱερὰ γράμματα
οἶδας, τὰ δυνάμενά σε σοφίσαι εἰς σωτηρίαν διὰ πίστεως τῆς ἐν Χριστῷ Ἰησοῦ· (RSV "and how from childhood
you have been acquainted with the sacred writings ..."; NEG "dès ton enfance,
tu connais les saintes lettres, ..."). Odd that Turner is citing a perfect as an example of the present being used for the perfect; maybe he thinks that despite its morphology, it's really a present. At any rate, Timothy still knows, and the extended reference time is from his childhood. The French uses the present as expected.
2 Pet 3:4 καὶ λέγοντες· Ποῦ ἐστιν ἡ ἐπαγγελία τῆς παρουσίας αὐτοῦ; ἀφ’ ἧς γὰρ οἱ πατέρες ἐκοιμήθησαν, πάντα οὕτως
διαμένει ἀπ’ ἀρχῆς κτίσεως. (RSV "... For ever since the fathers fell asleep, all things
have continued as they were from the beginning of creation."; NEG "... Car, depuis que les pères sont morts, tout
demeure comme dès le commencement de la création."). The continuing is (still) present and two extended reference times are in view, both reaching the present.
1 John 2:9 ὁ λέγων ἐν τῷ φωτὶ εἶναι καὶ τὸν ἀδελφὸν αὐτοῦ μισῶν ἐν τῇ σκοτίᾳ
ἐστὶν ἕως ἄρτι.(RSV "He who says he is in the light and hates his brother
is in the darkness still."; NEG "Celui qui prétend être dans la lumière, et qui hait son frère,
est encore dans les ténèbres.") Unclear why this is cited. It's a standard present, though we do see an extending from the past "up to now."
1 John 3:8 ὁ ποιῶν τὴν ἁμαρτίαν ἐκ τοῦ διαβόλου ἐστίν, ὅτι ἀπ’ ἀρχῆς ὁ διάβολος
ἁμαρτάνει. ... (RSV "He who commits sin is of the devil; for the devil
has sinned from the beginning."; NEG "Celui qui pratique le péché est du diable, car le diable
pèche dès le commencement.") The devil is still sinning, and the extended reference time is from the beginning. The French, again, uses the present.
My sense of this grammatical category is that it covers cases where English uses the perfect when other languages are content to use the present. This is such a strange category to use in a Greek grammar (though textbooks may want to point it out).