The best practice is to take the FG when you are planning to do something with the information.
For example, if it's day 10 and you want to bottle in a few days, take an SG and then take another one in three days so you can ensure it's done. Or, if it's day 15 and you want to rack to a clearing vessel and dryhop, you may want to ensure it's done.
I generally package my beer by day 15 or so, so I take a reading then to make sure the beer is finished and clear.