With that small of a tank I'd do that about twice a week. Test your water doing that and see what parameters are to make sure you keep ammonia and nitrites to 0 and nitrates shouldn't get too high (maybe 10-20ppm). Also when the shrimp start breeding you will need a bigger tank, the general rule for them is about 10 shrimp per gallon so you could be overstocked very quickly. On my 10 gallon I do about 10% weekly or even a bit less often as its not stocked very much (about 30 shrimp and some snails). That's why I say do it more often on that small of a tank.