The short answer is that nobody knows how often you will be called for a PIP medical assessment. Renewal claims are sometimes decided by the DWP, without being referred to health professionals. However, some claimants will have to attend another face to face assessment.
PIP awards are for fixed periods. Many are short-term, lasting two years or less. People with conditions that are unlikely to get better or worse get awards of between 5-10 years. From Summer 2018, the DWP announced that claimants who are awarded the highest level of award under PIP - and who have severe or progressive conditions where their needs are expected to stay the same or increase - will receive an ongoing award of PIP with a 'light touch' review every ten years.
So, many claimants begin with a two year award and at the end of this period, if there has been no change they may move on to a five to ten year award. This could mean just having two face-to-face medical assessments in the first 7-10 years of an award. But it is likely to depend very much on the type of condition you have.
Claimants with conditions such as depression or ME may find themselves on a succession of short-term awards because the decision maker may believe that an improvement could occur at any time.
Others with very severe conditions may find that they never have to have a face-to-face medical at all, but are simply assessed on medical evidence provided by a GP or specialist.