All the advice that I've seen, from lawyers to VCs, suggests periodic vesting. You want to have shares vest in equal increments every day, week, or month. Any longer than that and you're asking for trouble, like pre-vesting firings and the resulting bad-faith lawsuit.

The whole idea is to make vesting proportionate to work. So you don't want full vesting on day one. Your co-founder could quit on day two and leech the next few years of your hard work. (Plus any VC is going to make you revise that.) But you also don't want someone putting in three years of work and never vesting.

I would probably go with monthly to make it fair and easy.