I’m testing 17 days on 4 days off with some modifications on day 17 & day 1.
I still can’t say anything conclusive though. Maybe it’s fine, maybe it’s not. You can try it yourself, if you feel that the burden is too much, you can always take a washout. More rest, but that’s the price of experimentation
I already did 3 days in the past, also 5 days and I am not sure If I ever did more than 5. But no matter how many days I took I did not feel any difference. Probably the difference is there but I am not aware of it.
I tried 3 and i could feel the burden, it’s a bit too little for me.
5 days feels like a more complete rest when i was doing 21 days on.
4 days of wash out, resuming on the 5th day.
That seems to be good for me.
I’ve going to do a wash out remainder of December beginning today. Curious how this will play out for me with bloom. 
I usually do a 4 day washout after each ZP cycle. Sometimes 6 days if I change my stack.
I once did a 15 day washout while on vacation. This washout was good. Very little recon experienced while having bloom effects.