I tend to train them twice weekly (hams the same). Once with an upper body muscle (usually chest) and the other in a full blown leg workout.
I can't squat due to issues with my back but been bringing them back in slowly recently. I have been doing light weight barbell or smith machine squats and kettleball squats on platforms. Otherwise squats would be the primary exercise.
As a result on mixed day it is mainly leg extensions to warm up. Then horizontal leg press and onto 45 degree leg press. I usually finish with heavier leg extensions.
A typical leg day for me would be something like this...
Standing calf raises supersetted with calf presses. 5 sets of each for 20-12 reps. I go up in weight every set and have minimal rest periods.
Donkey calf raises... 5 sets going up in weight using slow reps and holds at the top and bottom of most reps.
Lying leg curls... 4 sets of 20 reps going up in weight.
Standing cable leg curls... 4 sets for each foot for approx 15 reps going up in weight.
Seated leg curls... 4 sets ending with heavy weight and holding each rep.
Stiff leg deadlifts... 4 sets of approx 15 reps going up in weight
Hip Adductor supersetted with hip abductors. 5 sets of 2-15 reps going up to the full weight rack. I go up in weight every set and have minimal rest periods.
Leg extensions... warm up and lots of reps
Horizontal leg press for approx 10-15 sets and 20-10 reps using a variety of foot positions to target different areas.
45 degree leg press.... quad dominant and about 10 sets of 20-10 reps with minimal rest (10 to 5 secs) between sets.
Leg Extensions... 7 sets of 20-12 reps using static holds, slow reps etc. Sometimes I will superset these with sissy squats.
Then I crawl to the shower
I should note because of the sheer volume sometimes I even miss out calves on leg day because I pretty much train them every other day in the gym.