Presumably this is a VM that is not managed by a View Connection Server? You should be able to set/change the following registry setting (this would automatically be updated if you were using a CS):
HKEY_LOCAL_MACHINE\SOFTWARE\VMware, Inc.\VMware VDM\Node Manager\DisconnectLimitMinutes
A value of -1 means immediately, a value of 0 means never. Positive values are the number of minutes to wait after the disconnect occurs.
Mike