As soon as you hear them get dirty. i.e. scratching noises when they spin. You'll know it when you hear it.
As to cleaning them with lubricant, you can't. That would merely be lubricating a dirty bearing and make things worse.
What I've found works best is an initial wash with mineral spirits. Then a follow up with denatured alcohol. Repeat with the alcohol until no more dirt particles are coming out. Once dry (I usually use a heat gun to dry mine) make sure they spin free with no "scratchy noises." If so, then lubricate with your choice of lube. Tri-flow, Bones speed cream, RnR Rocket propellant....
BTW, unfortunately no bearings will last forever. But if you take care of them, you should be good to go for a while.