I'd check with your saddler, I get mine checked once a year.

 Every six months. 

I get my saddle checked twice a year from the saddle fitter i brought it from as my horse changes shape and has lost a lot of weight. Like your horse mine is not sound all the time and it means i am pushed more to one side when riding her which means i need to keep an eye out on the saddle to see if it needs reflocking on that side. If you ring a saddle fitter they can come out and have a look at your saddle and horse and offer advice on any work that may need doing to the saddle or if you just need to use a thicker nummah even they dont cost much money to come out and check.