Pretty much every workout. I think it really helps improve SDKs and core strength.
Here's a link to a prior thread where monofins are discussed and related threads referenced.
forums.usms.org/showthread.php
Monofins are great- they really give you a good feel for what a good underwater fly should feel like.
And if anyone would know, that would be The Fortress.
Regular fins are flexible. Monofins are hard and they keep your feet and legs together. You can't bend your knees as much, if you were tempted to, with a monofin. Because of this, monofins work the core more and simulate and promote the underwater undulation better. At least that's what I think.
I should also clarify that I don't swim fly with my MF, except for a one arm fly drill. I use it primarily for underwater kicking or shooters, 25s, 50s, every once inawhile something longer. Plus, it's just fun to swoosh through the water like a fish.
However, as ande would say, you have to practice SDKs without the MF too. He has a great SDK and he never uses fins or MFs.
I have a competition monofin. The main problem is that not all pools are willing to allow you to use them. My pool does but I swim early in the morning before work and there are usually many swimmers so sometimes it's just not practical.
My problem with monofin training is the let down when I take them off. All of a sudden I'm not flying anymore :(
Lately I have used zoomers - sort of going to the other end of the fin spectrum.
Lastly, I can't seem to do a back dolphin with my monofin.
