I check at the beginning of each long trip by removing the center cap. I also check the tire and hub temperature as I make stops along the way. If you use the back of your hand near the center cap as you walk around the unit you will notice any hub that is hotter than the others. Water submersion is the main reason to repack in my experience. I have gone through flooded areas and needed to repack right away and if I keep the axles out of the water I have waited as long as 20,000 miles and 5 years on the RV. On the boat I repack at the beginning of each season and check as mentioned above each time it is in the water.