I'm about 2/3rds done and running low on glass agitators.
every six months my mom and I do a water-and-shake. Open the bottle, look at the texture. If it looks thick (and with practice you can tell) add some distilled water. Replace lid tightly, and shake-shake-shake (whether you added water or not). Mom has about 300 craft paints so it takes a while.
So who else does this on a schedule?