25 ft of 1/2" copper long enough for a HERMS coil?

Planning to use a coiled copper HEX as a HERMs and IC. Is 25' of 1/2" good enough?

For five gallon batches? Definitely.

10 gallon.

I'm assuming that there is an equation kicking around that can verify it for me, but I haven't found it yet.

Ten gallon would be pushing it in my experience, but not useless. You'll just have to keep your herms tank a bit hotter than you otherwise would. That can actually be nice if you're trying to use your HLT as a herms chamber.

Actually 25' is more than enough - with a big IF.

If you keep the water in constant motion in the HLT. The heat exchange is dependent on shearing the water next to the copper line. Without doing this, there is an insulating barrier layer of water that builds up within less than a NY minute. Keep the water moving and the barrier layer is constantly stripped away. Doing it this way, the temperature between the liquid inside and outside the tubing reaches equilibration in less than 15 feet with 1/2" OD tubing. This also depends on a reasonable flow rate inside the tubing.