I've created a backup of my wallet.dat (in a TrueCrypt container, of course) and have it stored in 7 or 8 different places.

I read somewhere that if you go too long without updating your wallet.dat that it can become out of date and won't work properly anymore, something to do with only 99 receiving addresses being stored in the file. Realistically, how often should I update my backups to make sure they stay functioning? I intended on updating them roughly once a month. Is this not often enough, or is it more often than I really need to worry about?

I receive 1-2 payments a day, on average, if that makes any difference.