Six months, max. If you know, you know. That's the problem. When you don't know, you don't...and so you wait around, hoping one day you'll have that certainty. But when you don't know...you just don't. You shouldn't have to wait forever to know. How long did it take you to know when you were in love that once, five years ago?
Two months. That feeling held for about two years until depression took it away from me. Since then I haven't felt anything close to the same feeling. This relationship is at month 2.5.
Well . . . dating sucks balls but . . . there's worse things in the world than spending a bit of time with a kind, patient, talented person. Have you checked in to see how she's feeling about things? That might clarify or affirm things that you're noticing about your own reactions (or non-reactions).