How long is a piece of string?

Seriously, there's no way, just from the size of the DB, to make any educated guess as to the time to shrink. Depends on IO subsystem, amount and distribution of free space, type of pages (LOB, data, row overflow), usage of server, etc.

Before you consider shrinking, is that space likely to be reused on a reasonable amount of time? If so, rather don't shrink. Free space won't make your backups large or slow down queries and, if you shrink, you'll need to rebuild all your indexes afterwards.