You should be at final gravity by the time you put it in the secondary. If there was a rule of thumb, it would probably be 1-2 weeks, but it really just depends on what you want to accomplish. Conversely, there are things like wheat beers that don't really benefit from a secondary at all.
As a general rule of thumb for a standard brew, it is 1 week primary, 2 week secondary, 3 weeks in bottles.
As all rule of thumbs go, it depends. Some brews I leave in primary for two weeks to a month. Some brews I leave in the secondary for only 48 hours crash cooling it, or some of my braggots, and darker beers I keep them in secondary for months.