It depends on how much power you have, how smooth the ground is, and how rough or thick of stuff you are mowing. I have heard some people claim they mow 8mph but you would have to have some smooth ground for that or you would beat yourself and the tractor to peices. I think realistically 3-5mph(or at least for the rough ground in my area), you may be able to go faster in spots and you may have to go slower in spots. I have seen some places were you have to crawl like the a portion of the feild was plowed and nothing was ever done to it.
Check lanscapecalculator.com for their mowing calculator to get a idea of how long it will take. Don't foget you will not get a full 10' cut out of a 10' cutter.
Have you thought about a cab since you have so much mowing? It is really nice being able to stay cool and helps out on the alergies.