Depends on the audience you already have.
It's all a numbers game, say you want to be top 3 on a Tuesday. How long would it take you to be able to rally, let's say, 500 people to your launch? That's more or less how you can establish the timeframe you should work with.
@nicolaas_spijker Yeah 500 (or even higher) seems to be the requirement to get into top 3 these days. For someone with 0 followers, they'd need to build their fanbase months ahead!