I believe that there are two forces at work in declining length/intensity of a high: tolerance and loss of novelty. Tolerance varies from person to person; I find that my tolerance resets completely with just a few days of no smoking. However, even with no tolerance, I could never get the same type of high I got the first few times I smoked. The upside is that I am much more functional while high, which allows for a much deeper exploration of the unique mental effects of THC. While I wish I could experience those first couple times again somehow, I would rather keep the experience I can now get than give that up entirely in favour of those much more intense highs.

So anyway, to answer the questions: once a week is fine for most people as far as avoiding tolerance goes, and you could probably get away with much more frequent smoke sessions without major tolerance issues. As far as the amount of herb to use for a joint, it depends on a lot of things, most significantly the strength of your stuff and how high you want to get. In terms of grams, the normal range is about 0.7 - 1.4. If you're using potent stuff and smoking alone, start with a very small amount (just enough so you can roll it properly and it holds its shape without too many air pockets underneath the paper) and increase to taste. With schwag / low end stuff, I would suggest using significantly more - a good rule of themb for a schwag joint is that it should be thicker than a cigarette. The best advice I can offer is to experiment and find an amount that suits your needs.