Both imperfectives [imperfect and periphrastic] occur in Acts 12:5 (ὁ μὲν οὖν Πέτρος ἐτηρεῖτο ἐν τῇ φυλακῇ· προσευχὴ δὲ ἦν ἐκτενῶς γινομένη ὑπὸ τῆς ἐκκλησίας πρὸς τὸν θεὸν περὶ αὐτοῦ [NA28] “While Peter was kept in prison, the church prayed fervently to God for him”). The simple one (ἐτηρεῖτο) is consistent with Peter being kept continuously in the prison. The copular form (ἦν … γινομένη), being more stative, suggests that, while prayer was being repeatedly offered for him, it may not have been continual, around the clock prayer.<idle musing>
 “The word [ἐκτενῶς] has rather the idea that their prayer was earnest and fervent, than that it was constant” (Albert Barnes, Notes on the New Testament, III: The Acts of the Apostles [Glasgow: Blackie, 1846], 217) (emphasis original).
Hmmm...this is often used as a prooftext for 24 hour prayer vigils. If Levinsohn is correct—and I think he is!—then maybe we need to rethink the rational behind them. Maybe the duration isn't as important as the fervency and earnestness? I would say that is more consistent with NT principals than the around-the-clock view.
Isn't grammar fun? Nothing like a little Greek to upset your received theology, is there?