How long do you normally use the secondary fermenter before bottling? Is there a common rule of thumb for this or do you base it off of the gravity reading?
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.