I check my hubs for heat with my hand and for any signs of leakage every time I stop when traveling with my boat. We travel often and pull the boat up to 10 hours in a day. If I see any grease leaking out, I will stop and check for heat more often, lack of grease or if the grease has lost its viscosity (watered down) will cause bearing failure. I keep a spare, per-greased ready to install, hub with me at all times. I can replace the hub on the side of the highway if needed but have never had to do it. Once grease is leaking out, remove the hub and rebuild it with new seals. I have had to do this 3 times in the last 9 years and we put our boat in the water pretty much every weekend. The bottom line is: keep an eye on your hubs, be prepared in case one fails.