Water heater drain pan pump capacity needs?
It depends on what type of failure are you talking about. The rate at which water flows from a leak, is going to depend on the size of the leak. It could range anywhere from less than 1 ml per day, to the full supply flow rate.

If it's a catastrophic failure, you could be talking about the full contents spilling out in a matter of seconds/minutes, followed by the full supply flow. 

In the case of a T&amp;P discharge, it could be a tablespoon to a few cups. If the T&amp;P fails fully open (not very likely), it would be close to the full supply flow rate.

If you want to plan for nearly worst case, you'll want a pump that matches the supply flow rate.  In the event of a catastrophic failure, there's not much you can do about the initial surge. But if the pump can match the supply, you'll reduce further flooding until the supply can be shut off.

I don't have solid data on it, but I'd guess that serious failures are rare, and catastrophic failures are even more rare.  Therefore, matching the supply flow could be a waste of money (unless of course the tank actually fails).

An automatic leak detection shutoff valve, is another way to reduce damage caused by a water heater failure.  Though it won't do anything to prevent the water that's already in the tank, from leaking/gushing out.