90 minutes with a 21" mower is probably too much. You probably should have a small rider, like a rear-engine Snapper, about a 28" or 30" cut. The smaller ones can go through a 36" door, like on a shed.

Here is a link that might be useful: Snapper

My situation is exactly like bekellog's, except that I have some physical disabilities that are making it impossible for me to handle a "walk behind" 21" Snapper for an hour and a half, and I need extended rest breaks to complete the job, so it takes me about 2 1/2 hours, depending on the temperature.

My advice would be exactly like saxman1's.

I just last Friday, (7-2-10) visited a Snapper dealer and had him order a 28", 12.5 hp Snapper RER for me. It should be 1/3 faster for me, plus I should not need rest breaks with the rider, so I should finish in about an hour, and not need several hours to recover from the chore.

Cutting faster will inevitably mean cutting more grass with a bigger diameter cutting deck. There are larger walk-behinds and of course there are riders. However, before you go bigger, consider whether your yard will allow you to take advantage of a machine with a bigger cutting area. If you have to do a lot of backing and turning because of bushes, trees and other obstructions then you may lose time over the more maneuverable 21 inch mower.

Another issue to consider is ground speed. If the bigger mower has a lower top-end then you may not save as much time. This became apparent with a large walk-behind that briefly attracted attention on the Lawn Mower Forum as a close-out. Owners found out after the fact that ground speed was slower than a standard 21 inch because of the significant increase in volume of grass to process.

Check out the Sutech Stealth. Major drawback with this machine, No reverse. But very well built and a great cut quality.

Here is a link that might be useful: Sutech Products Page