I put the roller in a zip-lock baggy everytime I was done using it, kept it reusable for days. In fact it was usuable until I didn't use it for a few days, then it started to dry up. I think the longest I used one was 10 days or so. Yeah, I'm taking a long time to do it but with family and work and now winter, I'm doing it a piece at a time, which for white, shouldn't be an issue.

I have also just poured unused paint right back into the can, it didn't seem to cause any problems.

Gerbs