As long as you don't drain the water from the membrane's housing, you'll be fine. If you use it only once a week even, you won't have to worry. I have a 3 membrane system that I run as needed (which is about once a week). Have done this for a few years now and still have the same membranes still producing tons of RO water.
Now, if you remove the membrane from its housing for any length of time, then you'll want to seal it up in a Ziploc and store it in the fridge, that's your best hope.