Many glow plugs swell with excessive heat. Most just burnout with excessive time. Yes they are intended to glow but for a set amount of time. I have pulled several swollen plugs out of motor so I caution on the side of less time. Block heaters are cheap compared to pulling heads. Best yet is using high quality plugs.
Even down to zero temps with a strong battery and good plugs every diesel should fire up. Weak batteries and older plugs or misused plugs will make the job difficult. When I get my new tractor I will just ask for the block heater to be installed with all the other prep work. I currently have too many projects as it is.