I haven't heard of IHT based on 6 months on, 6 months off.
It's usually based on PSA levels, e.g. some level depending on onco and patient (such as PSA 10) triggers going on, and when it gets down to a low stable level for 6-12 months, you can come off. Most people on IHT spend longer off than on. Some mens' PSA rises too fast when they come off to be able to use IHT.