Temporal clauses referring indefinitely to the future take either the subjunctive with ἄν or the optative without ἄν.
a. The addition of ἄν produces the forms ὅταν, ὁπόταν; ἐπά_ν, ἐπήν (both rare in Attic), ἐπειδαν. ἕως ἄν, μέχρι ἄν, ἔστ' ἄν mean
b. The temporal conjunctions have here, in general, the same constructions as conditional ἐά_ν or εἰ. Thus ὁπόταν ἐά_ν ποτε, ὁπότε εἴ ποτε.
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].