Replace if the rubber begins to crack or peel.
Cracked or peeling gloves will not resist water and chemicals. Toss them and get a new pair.
Replace if the gloves begin to develop a bad smell or sticky film.
Sometimes even with proper maintenance of your gloves they will retain a bad smell or develop a filmy residue. If this cannot be cleaned without the smell/film returning, throw them away.
Replace if the lining becomes worn.
We want a liner to make gloves easy to get on and off. The lining also helps absorb moisture, making our hands more comfortable while wearing them.
Replace gloves if the gripper surfaces have worn off.
The gripping surfaces on palms and fingertips of gloves are essential to making sure we can hold onto whatever we are cleaning. If these have worn off, you've put the gloves to good use, and it's time to put them out of their misery.