If you do not have an adsense account there is no point in signing up until you have a site they will approve and can show some traffic. If you sign up too early then you will get rejected and have to reapply... thus wasting your time.
There is not one general rule of thumb as to how much traffic is enough to sign up. But there is little point in having an account with low traffic because you will only make pennies.
The average webmaster makes $50 a month with Adsense. The average clickthrough is 1-2%.
So if you are new and only have 100 impressions a day on average you may get 1 or 2 clicks that pay a few pennies each.
If you have a lot of high quality traffic your payout goes up. Depends on niche as well.
But Google will determine what is high quality and what is not. And new accounts need to be "seasoned". So get some good traffic then sign up as soon as you have it.