I'm assuming you use wood chips. So...when wood chips start getting dark brown to black, or they getting mushy and break down, creating debris, then change it out. Usually orchids don't need to be potted up in size for a few years...unless the root mass is clambouring out of the pot.
I suppose that i was mostly wondering about the repotting cause i am used to my phaleonopsis' needing to be repotted once a year when they are done blooming and the oncidium has just finished blooming. So i wondered if the repotting is contingent on blooming?
I'd repot AFTER blooming personally. If I have to. They do fine somewhat root bound. I re-potted most of mine this year, won't need to again probably for 3 years I'd guess.