Jay Brittain of Jay Brittain Plumbing, (603-520-7713), replaced my disposal a couple years ago. Pretty quick, as I remember...maybe a little more than an hour, if that? If you have septic, make sure to get the unit that is septic-compatible, (has a cartridge of enzymes that are released with each use). There are also different horsepower units...the bigger the garbage you grind up, (e.g., chicken bones, etc), the more HP you need.
Regarding sink replacements, drop-in sinks go reasonably quickly. Undermount sinks can be a little more time consuming. Of course, with sink replacements come the style & function decisions of stainless steel? ceramic? New fixtures?
And of course, once you replace the sink, you'll need to replace the countertops, cabinets, appliances, floors, etc, because they will all need to coordinate....
