The 4's and the 8's: Estimating Work Orders Wrong

This week I have been teaching asset management principles and as a part of that we were discussing the work execution process including planning of maintenance work and the schedule that follows. I shared with the group that my experience tells me than many if not most work orders are slotted into half and whole shift time estimates for the schedule. A gentleman from a very large facility chimed in and added some data to the mix. He said within his facility currently 88 percent of the work orders in his backlog were estimated at 8 hours. 6 percent were 4 hours and the remainder were at random intervals.They were half and whole shift...
If you can only achieve that level of accuracy then are the jobs really planned?
The planning process should include breaking down the job into smaller steps where estimation is easier and more accurate. If you are not breaking that job down into the component task then your inherently inaccurate job plans will lead to inaccurate schedules and that will provide for wasted craft time, excessive downtime, and broken delivery promises to operations. Job plans in general do not have to be planned to the minute but 4 hour and 8 hour is definitely not enough.  
What level of planning accuracy would you suggest works for your facility?
to the hour? to the quarter hour?