I have had this RBTA for a few months now and my clowns are hosting it. It is pretty big and if it would split. And today I noticed a second mouth on it which I think means it is splitting but if it is splitting how long should it take?
I had one take about 24 hours once. you can also split them manually if you are interested. I believe the length of time it takes can also depend on how strong they are and how sharp the rock ledge they find to use is.
Do you have another saltwater tank set up? If so, try trading him out to the other tank. Often the stress of moving around (different flows, lights, very slightly different salinity) can cause them to finish the job faster.