Excellent points and questions! These kinds of discussions are why I think it's so important to not only debate different translations but how to interpret the original language as well. This is just a first response. I'm not addressing directly yet your "How much time DO I need?" but that's important as well. The only thing on that which comes to mind is the beginning of the letter to Menoikos: Let no one put off the love and practice of wisdom [note] when young, nor grow tired of it when old.That doesn't address duration but only "Don't waste it while you have it!" I'll cogitate some more on this one.
To address the "best life," your question made me curious if the phrases ἀρίστου βίου "the best life" and παντελῆ βίον "the complete life" show up anywhere else in the works, essentially asking Epicurus to define his terms (although he may dislike that characterisation).
Interestingly, as far as I can tell, those two phrases only occur as phrases in PD 20. However, παντελῆ shows up one more time in the next PD:
PD 21: Ὁ τὰ πέρατα τοῦ βίου κατειδὼς οἶδεν ὡς εὐπόριστόν ἐστι τὸ <τὸ> ἀλγοῦν κατ’ ἔνδειαν ἐξαιροῦν καὶ τὸ τὸν ὅλον βίον παντελῆ καθιστάν· ὥστε οὐδὲν προσδεῖται πραγμάτων ἀγῶνας κεκτημένων. Hicks translation: He who understands the limits of life knows how easy it is to procure enough to remove the pain of want and make the whole of life complete and perfect. Hence he has no longer any need of things which are not to be won save by conflict and struggle.
Here we also again find the "limits of life" (τὰ πέρατα τοῦ βίου) as well as "the whole of life complete and perfect" (τὸ τὸν ὅλον βίον παντελῆ). So, from this, I interpret the "complete life" to be described by Epicurus as one in which one "has no longer any need of things which are not to be won save by conflict and struggle." One is maximizing pleasure and minimizing conflict and struggle. That's the best life. And we achieve it through applying the Canon.