|When my DS#1 was small, I would rinse his covers under the tap and air dry, but only wash them in the machine once a month or so (or if soiled and handwashing didn't quite get it). That worked fine until I weaned him and he started peeing a lot less, but more concentrated, and the rinsing thing just stopped cutting it. Then I went to using one cover per day and washing with other clothes. That was also in the L size, and I knew he'd be in that forever, so I went ahead and bought more covers so I had 8 and didn't have to worry too much about getting the dirty covers into any going wash load.
You'll know if you need to do something else, but for now, stick with what works! :-)