Front bearings can last a season if cleaned and re-oiled after every race. For rears, other than being dirty, they will start to show excess play side to side and up/down. Its typically a good idea to start the year with fresh bearings and replace them again halfway through the season regardless of condition.
My rear bearings get replaced every 8 races whether they need it or not. I keep a couple of good used ones in case someone brings their kart to my shop and their rear bearings are shot. The used ones are free BTW.
Don't forget to check your spindle bearings once in a while also. I've seen some horrible spindle bearings come through my shop.
Front bearings are sealed, I leave them that way. After 10 races last year I only had 1 that was just starting to get crunchy. The rear bearings I use have a metal shield, rather than rubber seal. I hit them with brake cleaner, blow dry, and re-lube after every weekend. Had no issues with rear bearings last year. All get changed at the start of the season regardless, but I also keep spares in the trailer.