If you want to commute, do a trial run on the weekend, if traffic is involved add loads of time extra on and just slowly decrease the extra you give.
I was giving myself 40 minutes moving at first, as I got faster and more used to the timing I actually started increasing the distance a bit so I still leave at the same time.
My commute is pretty much bang on 10 miles. I usually work on the principle that it takes me exactly an hour to get from my home to my desk, including getting caught in (fairly limited) traffic, locking bike up in the basement carpark, and getting changed.
It takes me slightly longer in this weather to be honest though. I tend to just pootle in enjoying the surroundings.
I give myself an hour at the point I leave the house (getting the bike out the garage too etc). Although I regularly get to my desk a good 10-20 earlier, and gives me a bit of time in case I leave later. Moving time, never more than 35 minutes for 17km (16.09km is 1 mile).