People are kind of busy, and sitting down to write a letter is something that takes way more active choice and perceived work than doing... well, anything online. Once every week might be too often (or maybe not? Maybe it's perfect, and lets people play a lot - depends on how fun and interesting it is), and a month is probably too seldom - I think I'd forget about the game in between "turns", even. Once every two weeks sounds nice, but I suppose the frequency kind of has to be iterated on depending on how people play, how into it people are, and those kinds of things.
Then there's the problem of mail arrival times. A letter from me to Canada takes 4-5 days, and I imagine it can take even longer from other parts of the world. That's kinda got to be in the equation for letter frequency and game design.