As much as you need, when you need it
I do a daily review (~20 min) and have learned that my life is just a bit too busy for the weekly review. Instead I do the equivalent of a weekly review every five days. Yes, that is even more often than normal. When things get really rough, I tend to do the equivalent of a weekly review every other day (or even everyday) to stay clear and calm. You review as much as you need to to stay clear.
That said, I have given my self permission (sometimes) to go for up to three weeks without any kind of review. For example, when I'm traveling I get tremendous amounts of input and I would have to be constantly in review mode to keep my system up. Instead, I keep around several notebooks, digital camera, portable inbox, phone, etc. and just use everything as in. I'll sort it all out over several weeks after I get home.
Ps. I'm from Idaho and run an small but international company out of a small rented apartment in Brazil. I live with my not bi-lingual wife and have two kids under the age of four. And currently, I'm moving across the country to Curitiba. And even with all that, I'm clear enough to write this message to you. GTD is pretty amazing.