Had a quick wrist/grip workout today, and my earlier guess of twice per week still feels about right. In order to keep it interesting I've split the exercises over the two sessions, and will vary them in accordance with the way I feel on the day.
Wrist roller 3×5 @ 7.5kg
Grippers holding 2.5kg plate for max time, x 5
Plate curl 3×5 @ 5kg
Bar raise using standard barbell, x 5
Isometric stretching
Total time : 28 mins