100 ranks, but only for invasions/spars with rangers or other folks that cant disarm you, fact is you should never not have an arm worn shield on if you're trying to block due to disarms, and you should probably not bother with anything but armworn for training if you just want to train more shield.
However you CAN use a tower shield any time if it suits you, never let whats best statisticly replace what you enjoy doing.