Cautiously in flatwater at 8 1/2 months with a wrist splint .
I started this as when I Googled nothing much came up so in case this comes up for someone else in a similar situation I'll mention my experience.
I tried sailing at the 4.5 month mark. The wrist wasn't hurting to do household chores but only had 30% strength compared to the other and not much arm muscle build up. The hand was very sore the next day ( 9/10 pain trying to turn a light on) but started to come good after a day or so.
I then stuffed it up by deciding to speed up the rehab by doing my own version and really getting into it..I hadn't weeded with that hand or squeezed a ball but I did 30mins weeding them c 30mins whilst a passenger in the car squeezing the ball. I was also pushing through finger pain doing 500gm weights. .Anyway I inflamed the muscles in the arm and it took 6 weeks to settle and manage to get back into rehab.
Strengthwise I'd gone backwards because I'd stopped rehab.
Lesson learnt- they go slowly for a reason! Don't try and speed things up.
After 6 weeks at the 6 month mark I had another go. I was having hand pain doing household tasks and only had 45% strength in the bad wrist/hand. I had managed to do 12kms kayaking and could cycle and bushwalk so I thought I'd give it a go.
I was going out in subplaning conditions with a 7m on my 112ltre which is really too narrow and small to comfortably sail in slop underpowered. A gust hit and my wrist got a big jar 8/10 pain so I went straight back in and decided I wouldn't try again until I could push myself off the floor with it and it had strengthened a bit
So about 6 weeks later 8.5 month mark tried in flatwater in a Seabreeze with the wrist splint. First try was sub planing longboard type sailing. Today I finally overcame my fear of planing and had a ball! Very rusty and I suspect I'll pay for the fun tomorrow with a very sore and swollen hand but on the way back.
Ideally a large floaty board with a little sail in flatwater would make getting back into it easier but I don't have that option.
It's actually amazing how the body comes back if you stick to rehab and wait for it to heal.
I'm in my 60s so a younger person might heal a bit quicker.