Both ich and velvet have a resting stage, called a tomont. For ich, the ability for a tomont to remain infective has been shown to reach as much as 75 days. Velvet is less known. That extreme amount of time is only under very specific conditions - lower temperatures and few bacteria. A more typical amount of time is around 45 days. Here is a discussion about that:
I've been doing some research into the "76 day fallow period". I never heard of that prior to coming to Reef2Reef this past summer. I had always used 32 days as the maximum time for a tomont to remain viable under tropical conditions, and then I would round up to 45 days to help ensure that...
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Jay
