There is no limit for every application, so to speak. The amount of available memory for the application depends on the amount of free memory, which in turn depends on the amount of memory used by applications running in the background. These applications include constantly running system applications, such as SpringBoard, sometimes running system applications, such as Safari, iPod, etc. (And when iOS 4 will ship for the iPad) custom applications that are still running in the background.
However, I would say that an application should never use more than 50% of the total available bar. On an iPad, this currently means 128 MB and should be pretty much. Have you checked the leak check in your application?
Max seelemann
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