I think you do it when you get tired of brushing and combing every day! LOL If you don't have a show dog, there's no reason I know of to keep them in a really long puppy coat (other than the cuteness factor).
Cali is an mpoo and I finally gave in and got her clipped down at about 7 months of age. If I recall, I had about an inch left all over. She hadn't gone through her coat change yet at that point and, once clipped down, it only took me about half an hour once a week to keep her tangle free. She's pretty much finished her coat change now (she's just over a year) and her coat is really thick. I'm working towards a lamb clip, shorter on the body tapering into longer hair on the legs. I now spend about an hour twice a week brushing and combing. If I'm really busy, I can get away with once a week but will find a small mat or two usually in her legs.
I remember stressing over how much to cut off when I finally had Cali cut shorter. One thing I've learned since then is that her hair grows so darn fast that difference between a too-short clip and a perfect-length one is about a week.