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.