If it's dry, then it will be mostly dead. While it may take less than 6 or 8 weeks to be NH3 free, it will likely take a lot longer before it functions properly as live rock (denitrifying bacteria in the pores need to build up).
Another concern is much of the organics have died, dried up and are phosphates caches now. Adding water could result in elevated PO4 with the associated problems.
Old dried up LR is often not the deal it first appears to be..
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Brad
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