When I first started pheresis, August of last year, I went five times in a two week period. I then made it five weeks before needing another series of three. Since then, I have an order to go every three weeks. Every time that i hit the two week mark, I could feel myself getting weaker again and once I had my treatment, I bounced back quickly.
The last two months, I have been able to stretch my treatments out a little longer and I am now going every four. Obviously everyone responds differently, but with time, I am guessing you will be able to go a little longer between them.