They say you should clean it after every ride. I don't have the time or energy to do it more then once a month or so, unless it's show season then I try to do it before every show. -which sometimes is every week!
Dont let your leather get to soft or it could rip.
I use synthetics, so I wash them every time, which is easy cos I just scrub it and towel it dry. I don't wash my saddle tho, just wipe it if it has sweat on it. And any girths or breastplates get rinsed and hung up to dry.
My show saddles and bridles get polished after they are used or otherwise every couple of months to keep it moist. And I use a black polish so that it keeps the leather ( I use all black tack) looking like new.
I'm breaking the rythm... I will wash off (take a dry rag) to my saddle every few months, but I only use saddle soap, neatsfoot oil, and leather new when I'm doing a MAJOR cleanout of my tack room (like I did yesterday... lol)... I do that about twice a year...