If you spot clean the substrate as you go it can last months. Its all a personal decision on when to change it.
For Vegas, he poops a lot so I end up replacing his once a month or so. My smaller sulcatas just got a partial substrate change after 6 months of good use.
I have a very clean leopard. I have never seen poop yet in his enclosure, except his water dish. He usually does his potty duties in his daily soaks. I have had my coir since before Xmas. I just clean up the old food daily. I will be changing it out soon, only because I have seen a few bugs in it. I can't do bugs in my house. The flying ones are at least gone. So, I guess it depends on how clean they are, whether you can just spot clean for a long time, or if you have to change it up more offer.