Hi! I think it depends which plan you're under, but I'm a plan 2, and I haven't paid a penny towards student loans in the 7 years I've been here in the US. It's a bit of a pain doing the reassessment every year (it happens in June, and you don't need to wait for the letter that tells you they need info), but worth it besides. You'll need some proof of your income; paystubs, letter of employment detailing how much you earn, or an affidavit stating you don't currently earn anything.
If you don't provide info then they charge you a generic predetermined amount of a couple hundred a month i think, so definitely don't do that.
I'll probably have to pay something towards it this year though as I think I'm finally just above the threshold. :')