i bought the take home kit from my dentist, consists of 4 tubes of whitening gel and a mould. it's more convenient i feel and you can do it whenever you feel your teeth getting yellow..
was told to just wear it 1-2 hours each time, and after wearing it for 3-4 times my colleagues noticed a difference
i wore it on for 2-3hrs each time though hahaha!
i think the kit can last me for at least half a year as i have stopped wearing for 2 months now.