The only way you won't look needy if you're not. If you want to talk to her. Talk to her if you don't then don't.
It's what you want...
Be a fucking man and control your life not let her control it. This is all she wants from a man. Yes if you start your convo with her twice a day you will give out the needy vibe. But then again you need to find something to do if you need to talk to her twice daily
I don't want to be pain in the ass but "how long" is almost always a stupid question. I'm quite successful with women and it doesn't matter if you sms her 1000 times a day and respond the same second you get her SMS or if you let her wait 3 days. If you're not needy then she'll know it (and other way around). Just do what feels right.