I don't really budget, per se. But I don't buy fabric much anymore just because I like it. I try to get fabric with a particular project in mind. And I'm pretty slow, so my fabric purchases are spread out. My only impulse buys for fabric are fat quarters. I like to pick those up just for fun.
But I'd think budgeting for quilting would be like any other expense. Figure what your income can afford and stay with it.