How long do I temper my knife for?

It depends! What kind of steel do you have? Your best option is to look for any instructions that came with the steel, and failing that, to simply Google, “steel type heat tempering (temperature/time)”. As a rule of thumb, most tempering will be around 450°F for about 2 hours. Because you’re trying to achieve a certain Rockwell hardness, you could always get an instrument to test that as you go.

Here are some good places to start for common steel types: [1] [2]