I found some Toms Hardware article that goes into "a few GPUs in a desktop" all the way through "renting 20 machines with GPUs from EC2 for a while for <$20 USD", and it seems like a password that is long (>=12 characters for now), doesn't have dictionary words, and uses more than just [a-zA-Z0-9] will be safe from undedicated adversaries for a number of years (probably the life of whatever router you're using).