I have found that repair jobs like to 2-3Xs longer than I think they will. If I X3 my estimate to adjust for this the job will take 6 to 9X longer. So I struggle on in ignorance, hope for the best and once in awhile something gets done faster than I expect, but I am still always behind schedule!!!?
When I was in my twenties I would correct for this by working 20 hr days to catch up, but now I just accept it as the price of doing business and sometimes hiring a bunch of young guys to hustle it through.