I know this is a subjective question. As far as I know, there are somewhat about 25 MB available for the app, but it depends on what else is going on. Currently playing music, a current phone call or what ever might drop that amount of memory down a lot. I don't know.
Just tell us what you think, or what you have experienced. My app currently consumes about 15 MB at a time, mostly lots of images loaded into views for animation. I have encountered no problems with this. But is there some "border" that can be drawn, to make a simple-to-follow rule like "do not consume more than x MB of memory at any given time", where x stands for the max amount?