There are varying opinions (a couple in a very recent thread I'm afraid I don't recall offhand), but from my perspective I think if you don't have a good team yet you should feel free to roll as soon as you get 3k until you feel like you do.

What a "good team" constitutes may mean something different to you than it does to someone else, as a person who rerolls new accounts a lot I often see 2 decent heroes come out of the first gacha and then go "Alright, I'll play 'till the second 3k gacha and if I get complete garbage I'll delete it and start over."

On an established account though, I'd say wait until there's a particular Event girl you want and roll to try and grab her while the rate is increased. It's still terrible odds, but it's at least something.