Frequency of Full GC in Java

Hi,

How often should Full GC run? We have an issue in our MII landscape. A full GC wont run for two days and the the heap memory goes up to 90-100% resulting unavailability/slowness of application. A full GC then occurs and clears the the HEAP memory and bring it down to ~30%. Has anyone else faced this? We started to get this issue on our production  boxes all too often.

regards,

Rosh