About 2 in every 7 days - when I feel like my past night's misadventures might make it hard for me to sleep and wake properly. So no, not really. If I lived alone, I'd probably be more regimented and go for 7/7 with some tiny dose (because stable habits that can be tweaked gradually are reassuring).

Based on my experience, I'd recommend it if you want to wake up "naturally" with the light but your recent sleep history (e.g. jet lag) wouldn't permit that. Otherwise, I've been too sloppy to know precisely what it does for me (I can feel a large dose, such that I'd guarantee it's not only placebo).