If all the energy used by my 25 cu ft refrigerator was in turn used to heat hot water, how much water could?

Infinity amount of hot water, though it may take infinite amounts of time before it would heat all of it appreciably, and you'll need perfect insulation and some other stuff.

Essentially, the question is nonsensical. Sure, the energy used by your fridge could instead be used to heat water, but "how much" doesn't make sense. If you have a large, full kettle of water on the stove, it takes longer to heat the water than if you had a small, half-full kettle of water on the stove. The energy used by your fridge would be the stove in this thought experiment.

Not all that much if you go electric as i assume you're thinking of.

Do you plan on using a electric hot water heater they are very inefficient .

And the instant on units have severe limitations.

I'd try to cut down use a.m.a.p.

If you have a old 25 cu foot unit it's killing your bill so you'd see some possible savings.

But new Hi eff Refrigerators units are great.

I'd look for another way to save electric.

Call Sears or Home depot ask to speak to the (right person) & ask on new energy Eff. water heaters.

good luck.