I know that μακράν can mean either “far” (distance) or “long” (time), depending on context. But is the context here clear enough to make a determination?
Acts 2:39
It seems like most translations just have “far off” or “far away” without qualification of some reference to the future - https://biblehub.com/acts/2-39.htmὑμῖν γάρ ἐστιν ἡ ἐπαγγελία καὶ τοῖς τέκνοις ὑμῶν καὶ πᾶσιν τοῖς εἰς μακρὰν, ὅσους ἂν προσκαλέσηται κύριος ὁ θεὸς ἡμῶν.
ESV: For the promise is for you and for your children and for all who are far off, everyone whom the Lord our God calls to himself.
However, Culy & Parsons’ 2003 textual commentary "Acts: A Handbook on the Greek Text" (p. 31) has this translation:
Despite seemingly going out on a limb with this uncommon translation, their comment on this part of the verse merely says this:“For the promise is for you and your children and for all those in the future, as many as the Lord our God calls to himself.”
If the verse is meant as an allusion to LXX Isaiah 57:19, it would seem to tip the scales in favor of a spatial interpretation:τοῖς εἰς μακρὰν. The article functions as a nominalizer (see 1:3 on τὰ).
Isaiah 57:19
We see similar references in Ephesians:εἰρήνην ἐπ᾽ εἰρήνην τοῖς μακρὰν καὶ τοῖς ἐγγὺς οὖσιν. καὶ εἶπεν κύριος, «ἰάσομαι αὐτούς.»
NETS:
peace upon peace to those that are far and to those that are near. And the Lord said, "I will heal them."
Ephesians 2:13, 17:
BDAG seems to be on the fence about it, categorizing it as probably under definition 1.a., but giving definition 2 as an alternative:(13) νυνὶ δὲ ἐν Χριστῷ Ἰησοῦ ὑμεῖς οἵ ποτε ὄντες μακρὰν ἐγενήθητε ἐγγὺς ἐν τῷ αἵματι τοῦ Χριστοῦ
[…]
(17) καὶ ἐλθὼν εὐηγγελίσατο εἰρήνην ὑμῖν τοῖς μακρὰν καὶ εἰρήνην τοῖς ἐγγύς
Here are some of BDAG’s reference examples for definition 2:1. pert. to being at a relatively great distance from some position, far (away)
a. adv. of place
α. […] εἰς ἔθνη μακρὰν ἐξαποστελῶ σε I will send you far away to the gentiles Ac 22:21. […] The spatial sense, as Ac 22:21, is prob. expressed τοῖς τέκνοις ὑμῶν κ. πᾶσιν τοῖς εἰς μ. to your children and to all who far away Ac 2:39 (cp. REB; NRSV), but s. 2 below.
[…]
2. pert. to a position in time relatively far removed from the present, in time to come τοῖς τέκνοις ὑμῶν κ. πᾶσιν τοῖς εἰς μ. Ac 2:39 (cp. 2 Km 7:19 εἰς μ.=vs. 16 εἰς τὸν αἰῶνα; Sir 24:32 ἐκφανῶ αὐτὰ ἕως εἰς μ.—εἰς μ. also Demosth. 18, 36; Polyaenus 6, 7, 1; Jos., Ant. 6, 278; 20, 153; TestSim 6:2; Just., D. 2, 5; PMeyer 66, 2 [ostracon]), but s. 1 aα above.—DELG s.v. μακρός. M-M. TW.
The spatial sense, as Ac 22:21, is prob. expressed
2 Samuel 7:19 (εἰς μακράν = vs. 16 εἰς τὸν αἰῶνα)
Sir 24:32καὶ κατεσμικρύνθη μικρὸν ἐνώπιόν σου κύριέ μου κύριε καὶ ἐλάλησας ὑπὲρ τοῦ οἴκου τοῦ δούλου σου εἰς μακράν, οὗτος δὲ ὁ νόμος τοῦ ἀνθρώπου κύριέ μου κύριε
NETS:
And it was very small before you, my Lord, O Lord, and you spoke of your slave’s house at length, but this is the law for the person, my Lord, O Lord!
also Demosth. 18 (On the Crown), 36;ἔτι παιδείαν ὡς ὄρθρον φωτιῶ, καὶ ἐκφανῶ αὐτὰ ἕως εἰς μακράν
Still I will again make education enlighten like dawn, and I will shine theme forth to far off.
Polyaenus, Stratagems 6, 7, 1;τί οὖν συνέβη μετὰ ταῦτ᾽ εὐθύς, οὐκ εἰς μακράν;
What was the result—not the distant, but the immediate result?
Josephus, Ant. 6, 278;ἀφεθεὶς Ἀπολλόδωρος οὐκ ἐς μακρὰν τὴν τυραννίδα κατέσχε καὶ μάλιστα δὴ τοὺς ἀφέντας δικαστὰς ὠμῶς ἐκόλασε τὴν σωτηρίαν ἀναφέρων εἰς τὴν ἑαυτοῦ στρατηγίαν, οὐκ εἰς τὴν ἐκείνων φιλανθρωπίαν.
Not long afterwards, Apollodorus pursued his schemes with more success, and seized power. The first act of his tyranny was directed against the judges, who had acquitted him. He punished them with the greatest cruelty, as if he owed his life not their humanity, but to his own conduct.
ὁ δὲ βασιλεὺς ἐπῄνεσεν αὐτοὺς χάριν ἔχειν ὁμολογήσας τὸν ἐχθρὸν αὐτῷ μεμηνυκόσι, καὶ οὐκ εἰς μακρὰν ἀμείψεσθαι τῆς εὐνοίας ὑποσχόμενος αὐτοὺς ἔπεμψε τοὺς ζητήσοντας τὸν Δαυίδην καὶ τὴν ἐρημίαν ἐξερευνήσοντας, αὐτὸς δ᾽ ἀκολουθήσειν ἀπεκρίνατο.
So the king commended them, and confessed that he had reason to thank them, because they had given him information of his enemy; and he promised them, that it should not be long ere he would requite their kindness.
Josephus, Ant. 20, 153;
Testament of Simeon 6:2;Νέρων δὲ τὴν ἀρχὴν οὕτως παραλαβὼν Βρεττανικὸν μὲν ἀδήλως τοῖς πολλοῖς ἀναιρεῖ διὰ φαρμάκων, φανερῶς δ᾽ οὐκ εἰς μακρὰν τὴν μητέρα τὴν ἑαυτοῦ φονεύει, ταύτην ἀμοιβὴν ἀποτίσας αὐτῇ οὐ μόνον τῆς γενέσεως ἀλλὰ καὶ τοῦ ταῖς ἐκείνης μηχαναῖς τὴν Ῥωμαίων ἡγεμονίαν παραλαβεῖν.
And when Nero had thus obtained the government, he got Britannicus to be so poisoned, that the multitude should not perceive it; although he publicly put his own mother to death not long afterward, making her this requital, not only for being born of her, but for bringing it so about by her contrivances that he obtained the Roman empire.
Ἐὰν δὲ ἀφέλητε ἀφ' ὑμῶν τὸν φθόνον καὶ πᾶσαν σκληροτραχηλίαν, ὡς ῥόδον ἀνθήσει τὰ ὀστᾶ μου ἐν Ἰσραὴλ καὶ ὡς κρίνον ἡ σάρξ μου ἐν Ἰακώβ, καὶ ἔσται ἡ ὀσμή μου ὡς ὀσμὴ Λιβάνου, καὶ πληθυνθήσεται ὡς κέδροι ἅγιοι ἐξ ἐμοῦ ἕως αἰῶνος, καὶ οἱ κλάδοι αὐτῶν ἕως εἰς μακρὰν ἔσονται.
Now, if ye remove from you your envy and all stiff-neckedness, as a rose shall my bones flourish in Israel, and as a lily my flesh in Jacob, and my odour shall be as the odour of Libanus; and as cedars shall holy ones be multiplied from me for ever, and their branches shall stretch afar off.