Hours are a good indicator of a lot of things but this might not be that good for glow plugs... they would be more closely linked to number of start-up cycles, as the glow plug sequence is automatic and occurs for various length of time at every start-up.
I know that for my tractor I normally have over 2 to 4 hours at every start. There are a lot of guys I know that will start and stop their tractors every time they get on and off for what ever reason...
So if your tractor has been started and stopped frequently, your starter battery electrical system, including the glow plugs, will have been used a LOT more than someone that dose not frequently start/stop.
Any time that easy starts are a problem in cold weather... Glow plugs and control circuits are the first place to start looking. KennyV.