I am setting up a cigar lounge and plan on offering Friday night steaks to the members. I want to set up three tubs each with their own Anova’s. There will be a rare, medium and well done tub. I am wanting to keep the steaks in the rare tub and if someone wants a medium steak move it to that tub. If someone wants a well done then move it to the well done tub. My question is this. How long should it take for a steak that is sitting in the rare tub to reach medium doneness when dropped into the medium tank? How long for one moved to the well done tank?
I have never heard of anyone doing it this way,however, I can’t see why it would not work. The timing would depend on thickness of the steak and experimenting may be the best way to settle on timing. In your situation I would worry the patrons wanting well done may have to wait too long. I am guessing restaurants that cook steaks sous vide have some of each in each tub based on history. This way all they need to do is sear and season.
Theres definitely no reason why this wouldn’t work, however I’ve not seen any hard time/temp tables on what does work/doesn’t. Its really just raising the internal temp of piece of protein by a few degrees (5-6~).
The best way is obviously to experiment and post your results here of course ! 
The thinner the steak the faster the cooking/reheating time. I think heating from rare to medium would take just as long as from rare to well done.
Since your rare steaks are already out of the danger zone and not being heated from cold and pasteurized I think half the suggested cooking time is safe. But I am not an expert. I have read this but cannot remember where.
I would suggest 2 things.
Just have two baths one for rare and one for med and finish the well done ones on the stove/grill. One thread even suggested nuking them to finish the well done.
The other is buy a thinner cut for well done as the thinner they are the faster they cook. Most suppliers have various thickness already cryovaced in food safe individual packages.
One restaurant I know of started with just one bath. They finished the steaks on the charcoal grill and they were excellent. They did not advertise them as sous vide and they had a thicker crust generally but they got more consistent results and happy customers.
The big problem with the three bath system is what to do about the med-rare, med-well, cooked till it no longer wiggles orders. In the Canadian north the most popular orders are Med-rare and very very well done. The Inuit cook their caribou for 3 days and that is how they want their steaks. And that is how they like them in some southern states although I have no idea why.
Another problem is keeping track of when steak is switched. If you have one bath you would put in say 12 steaks marked with the date/time in case you had to ice bath them for the next day. Subsequent baths you would have to mark each steak as you moved them and keep track of when they were moved and might not be easy to mark wet plastic. Depends I guess on how many you plan to serve daily.
I think you could achieve reasonable quick results by having one bigger bath and two smaller ones which you could adjust the temperature quickly but sounds like a nightmare if you are busy.