I don't think it really matters as long as you're safe.
When i'm actually interested in someone, I usually wait longer. My last boyfriend and I were together for 4 months before we had sex. But i've also had one night stands because I know that nothing is going to come out of it and sometimes it just fun to have sex.
I need to have some kind of a mental / emotional connection to a guy before having sex.
I'm not giving it up after a week, but you won't be waiting three months either.
As a Cancer, i want to be emotionally connected with sb before it happens. I don't know for how long, i just have to feel something strong to have sex. It's always better.
Honestly I could live without sex and I'm all for waiting until marriage. Anyway I guess if 2 people really care about each other then it doesn't matter when.