How long does it take diving one place before you feel you have really begun to know it well?
Hard question. Some places, like the main west coast stretch of Bonaire, have a lot of fairly similar reef structure and life, compared to what I read about Roatan (where north vs. south are different, IIRC?). But even in Bonaire, a trip to the east coast 'wild side' diving can redefine the experience (e.g.: What? Sea grass?), and if you started out only seeing larger stuff, then later learned to appreciate anemone shrimp & other macro subjects, you might discover the place all over.
Compare Bonaire, a fairly modest-sized island, to the much larger Belize, where staying on the mainland vs. Ambergris Caye vs. Turneffe Atoll or live-aboard diving would give you much different experiences.
Speaking of which, is it just the diving you're concerned about? Or is it a solid comfort level for getting around the island, familiarity with the topside attractions & where to shop, eat, etc...? Is it necessary to stay at some different resorts for broader perspective, or is hitting the same one 10 times running just fine?
I like to get in close to 10 dives in a place during good weather conditions to get a comfortable sense I've 'been there,' but that's a far cry from being like it's a 2nd home, very familiar.
Summary: It depends on the size (e.g.: Bonaire vs. Belize), complexity/variety (e.g.: Roatan), # of dives you get in, and what you want to get familiar with. I agree that about 2 weeks in Bonaire gets you a good feel for getting around on-island, and the nature of mainstream west coast diving. But there's always more you might find.
Richard.