The multiple-hour window could give you a lot of wiggle room here though. It's unlikely to take 3 hours to restore from a single tape, so even if you have to visit 2-3 tapes then you have plenty of time.

I'm sure that there is a general tiered storage platform (as mentioned above) which keeps some of the data online as well. That would let you run a "defrag" algorithm later if you find you need it.