I usually find that they overestimate the time required. Issue with this is the pay, if a flat rate for each location it is great, if an hourly rate, then I always get screwed because I am more efficient and get the job done completely but quicker. Therefore I get paid less than people who aren't as efficient or pad their time for doing the exact same work and project.
When I started over 27 years ago, I was told by the Vice president of the very first company that I worked for to claim the whole time that the job stated. I.E: if it stated 8 hours and I got done in 6 hours, to report the whole 8 hours. As he said that the company is charging the client for the 8 hours and if you only report 6 hours then that is 2 hours" free money" for the company.
There should be a universal industry policy that if you can complete the work and maintain reasonable quality that you get paid for the job at the posted rate/time. If you are an employee your hourly rate would adjust accordingly. For example, if the job is 4 hours at $10/hr... But you finish it in 2 - then you get paid $20/hr for 2 hours. Same total pay, but the rate adjusts. Now if you do a crap job then you would get paid for the $10/hr rate and suffer the consequences of doing a crap job. The industry is really failing us.