Agitate for the first minute, and then 10 seconds for each additional minute.
Recipes & dev times using Developer Kodak D-76
Kodak T-Max 400 at 320 in Kodak D-76 stock
6 minutes, 10 seconds at 22°C/71.6°F
Kodak T-Max 400 in Kodak D-76 1:1
9 minutes at 24°C/75.2°F
Kodak T-Max 400 at 800 in Kodak D-76 1+3
11 minutes at 33°C/91.4°F