301 Redirects - how long do you need to keep the old domain alive?

[moehits]

When you 301 an old domain to a new one, and Google updates the links to the new site, do you still need to keep the old site live?

If you take the old site down, the next time they refresh the links, do the links still count to the new domain, or will they go away?
Do they recalculate the 301's every time they refresh?

[plasma]

You need the 301 as long as there are links pointing to the old domain.
I personally would never cancel a domain, it costs nothing, and you never know, what the next owner will do with it.

[g1smd]

You need to keep the domain going for at least a year - because that is how long it takes for Google to delist any supplemental results for the redirected URLs. Those links will still continue to deliver visitors so you need to keep it going.

You do not need separate hosting for this. You can point all of your domains at one server and have a large .htaccess file that redirects all requests that are for the "wrong" domain; only letting requests for one domain through to the content.