I wished i could use estimated time but it never works out for me. I may estimate that a project or task will take 30 minutes but then suddenly explodes and becomes half a day if not more.
For example, I'll estimate that changing a kitchen faucet should take 30 minutes. But then suddenly, a supply hose that is attached to the faucet is also leaking. Then the shutoff valve is rusty and now I'll have to get some tools that I don't normally have around. So suddenly this 30 minute project becomes 60 minutes because the scope has expanded suddenly.
I realize that time estimates may be useful but my life tends to be too fluid for me to even plan a day or week. What I do instead is to do a Weekly Review and choose 3 projects as my "Big Rocks of the Week." I'll flag these projects as projects I really want to advance significantly this week.
You can Google for "Big Rocks of the Week" as a strategy to planning your workweek.
http://www.google.com/search?client=...UTF-8&oe=UTF-8
Another time management trick I do is to just set aside blocks of time during the day to try to work uninterrupted on a particular project. I don't really use time estimates for this. I just try to get as much done as I can during these 30 minute, 60 minute, 90 minute blocks.
Interestingly enough, your project will often expand to the time you have allotted. You may have allotted a task or project for 60 minutes. But you may be working and let the time drag on for the whole hour when you could have just finished it in half the time.
So, for my particular situation, I don't really like using estimated time durations and have it hidden from my OmniFocus perspectives.