This is a good question, as there probably isn't an science to fast increases to capacity. There's a few factors that are perhaps predetermined (e.g. the way your belly is shaped) and then there are things like frequent practice. Frequently measuring your bloats, and bloating in the week should help (as you can see how much you fill up with, allowing you to increase the volume) and it can also help with stretching/staying full for longer.
I'd say regular inflation with air probably will help with results over time, letting your body get used to things like cramps and skin stretching as a physical feeling. So long as you're having fun and don't see it as a chore, go for it.
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