I have .com website which I don't want to use anymore (for a variety of reasons). I want to redirect the entire site to a .net domain with a 301 redirect. After how long can I cancel my .com domain entirely? What will happen to the linkjuice added by 3rd party websites which point to the .com domain? Will this be transferred to the .net domain?
In short, all of those 3rd party links, unless manually updated, will still point to the old domain. After you cancel it, they'll start getting 404's. You will loose all the SEO that's built up with it, and as LazyOne said, old bookmarks!
What will happen is you will let the domain expire so the 301 doesn't exist anymore, see a huge hit to traffic, and want to buy back the old domain to put the 301 back to get all that traffic back to the newer .net domain. However, the fee for this will cost you hundreds if not thousands of dollars.
And lets not count what happens if a competitor purchases it up.
$10 / year is not a bad investment to keep the domain when you come to think of any potential revenue and traffic (which should be turning into revenue!) loss that you will receive.