How durable are the saunas over time?

They hold up very well. Being a dry sauna, and operating at about 125° F, they tolerate heat very well. Still, howver, they are a natural wood product and may check or develop small splits over the years of use. The good news is that it won’t affect the function of the sauna and these cosmetic developments are easily repaired. As for the electronics, the bulbs are simple to replace if they burn out. Just unscrew and screw in a new one. And, while I have never had to replace a timer, it is an easy, inexpensive replacement that can be done in less than 15 minutes. Overall, the long-term durability is excellent and the maintenance is very simple.