I don't have any spotter when I train so I keep it in known limits for me.
Earlier this summer i was doing 50M every 3 minutes- including swim and rest time together. I could, easily, keep this up for an hour. If I'd had more time I would have done it longer. I really felt like this improved my diving. I did it with my shorter rubber hockey fins on.
In the past I have done 50M every 2 minutes when wearing my monofin- much faster swim for me. I would only keep this up for 8-10 sets but it worked.
Another drill I did was 50M repeats on a descending breath up scale. there would be 2:30 minute rest between the first two sets, but I would work my way down to just a :45 rest between the last two sets. This was almost like a C02 table, but with swimming.
When I have been out of it for a while, like the whole past month for me, i will start with 25M drills because these seem to be pretty easy for me and get me back into the work out mode without fear of burning out my motivation.
One last work out routine i stole from Hayden Welch who had it printed in Freediver magazine few years ago. It went something like this:
25M's every 45 seconds- 8 times
25m"s every minute- 8 times
50M's every 2minutes- 8 times
Swim hard for 20 minutes in fins- or monofin
This could be done in a lot less than an hour and you felt like you hit a little bit of everything when doing it. I have extended the surface swimming to round the work out up to somewhere between 60 and 90 minutes- depending upon how much time I actually have. I do the surface stuff with a front mounted snorkel and switch back and forth between dolphin kick and arm pulls- to try and work everything. This is the plan i will try and get myself back into in the next week- or so.
Jon