How often does one need to revise Project management plan?

There is no exact answer, because it depends on the length of the project, the complexity, the amount of change that happens during the project, including the staff changes, new processes coming along, and improvements from lessons learnt along the way. Personnally, I would routinely ensure that I would do a full review of it at least annually even if nothing on the project changed, just to make sure it is always up to date because things around the company always change in that timeframe.