There are no hard limits to how much and how often you can change your Avatar, aside from your level of power. Typically, major modifications of self should be limited to with-in your Heart. While you could theoretically completely redevelop your entire Avatar on a regular basis, this would interfere with your Keeper as a character. The general practise is to have a single major Avatar transformation, to upgrade your Keeper from your initial form to your full form, and minor modifications if necessary; or to stick with your initial Avatar.