When I use cypress mulch, (this is inside by the way) I just take it all out once a month and wash it. I put it in a big tub with holes drilled in and just spray it through thoroughly. I do the same with the spaghum moss. But at the same time I pick up all of the feces I see.
If you spot clean. You shouldn't have to change often, unless you are using substrate that is getting mold. I use coconut coir for my Leo and I change it out at six months to a year depending on how much is missing from the spot cleaning. If it still looks good and doesn't stink at six months I will keep using it. However, my tort does not poop in it, so mine might last longer then those who do poop in it.
Unfortunately I have no sense of smell. I do a spot clean everytime I see poop but they must be super private because I rarely see any.
Also I should note that I use the coconut coir and top it with a mixture of spaghnum miss and cypress mulch.
Besides the obvious health benefits of daily soakings, there is another benefit. They almost always defecate in their soak, and usually not during the rest of the day. If they do, mine usually go either on the border of or in their pond/water bowl (my hatchlings like to wade). Reptiles prefer to defecate in water; thats how you "litter-train" iguanas. So, if doing daily soakings, its unlikely to see much more feces.
My tortoise only poops in his soak. I've had one or two poops that were in his cage but the rest were during soaks. I use cypress mulch. Once a month I bake the substrate, then reuse. U have done this twice. I put it back in the enclosure, add a pitcher of water or two, and with the humidifier going it's completely moist within a day.