A general recommendation is to have 500MB per application. Each folder that is created in the webapps directory has its own context which means resources aren't shared, i.e. the the ROOT and samples directories.
The amount of memory required will also be increase depending on the number of users and size of requests. The current default value is 1,024 MB and can be changed by updating the registry value:
HKLM\SOFTWARE\WOW6432Node\Apache Software Foundation\Procrun 2.0\CWAppServer\Parameters\Java\JvmMx.