I consider water sterile when it has rolling boiled (top off) for 15 minutes. Then I use it immediately. I find the base of my Pari LC Plus' need a shot with the air compressor hose to get the water out of the small tube. My goal is for my nebs to only be wet for 30 min. Any longer and I'd worry about what Amy wrote, that the water was become non sterile on the neb and creating a breeding ground for bacteria. Prior to the rinse in sterile water I soak all neb parts in 3% hydrogen peroxide for 30 min. I have noticed steadier health since starting this routine a few years back.