When I first started quilting my instructor gave us this guideline:
If it's really shouting at you and you love, love, love it - buy 3 metres.
If you love it and think you'd like to make something out of it someday - buy 2 metres.
If you like it and just kinda think you may use it - buy 1 metre.
If it's just you're in the store and can't leave without buying something - buy a FQ.
Here, the fabric I buy costs around $20 per metre so buying 3 metres would be a substantial investment.