Good question Jim.
I think my instruction manual said something generic like "Backflush regularly". Hmmm, does that mean monthly, does that mean daily, etc.
Unfortunately, most RO/DI units don't come with the backflush kit even though you can order it as an inexpensive add-on. I found a flush valve kit here
Aquadirect :: RO Filters :: Kent Marine Filters :: All Kent Marine Products for $13.89 but you can get them so many other places (this is just the one that popped up first). If you have high TDS water where you live, adding this flush valve will greatly extend the life of your RO membrane.
With that said, the answer to your question is going to be different for everyone. My incoming water is at 350 TDS so I backflush my unit for about 1 minute at least weekly under normal conditions. If for some reason, I make more RO/DI water that week, I'll flush it again. If your incoming TDS was say 100, you could go longer without backflushing. However, I would recommend weekly just so it becomes a habit.
I'm now curious. How often does everyone backflush and for how long?