It's really hard to say exactly how long wood siding should last. There are so many factors involved, such as the type of wood used, the local climate, and how often homeowners paint the siding. Instead of operating on a hard-and-fast lifespan, it's better to just keep an eye on your siding and then replace it when you start to notice one or more of the following signs.
Rot is not always visible when you look at the siding from a distance. It may be hidden under paint that is loose but that has not yet peeled off. If you look at the siding more closely, though, you may realize that the edges of the cladding are black, or that there are bubbles of paint which, if scratched off, reveal black wood. Rotting siding can't do its job well anymore, and it may even attract insects, like termites and carpenter ants, so you should certainly replace your siding when you notice rot.
Sometimes as your siding ages, the wood either absorbs or lets go of moisture, leading to warping that eventually leaves gaps between the cladding. If this is happening in one spot, you can probably just have those few pieces of siding replaced. But if there are multiple gaps all over your home, this is probably indicative of a bigger problem with your wood siding beginning to warp. The warped siding with gaps between it will allow moisture to get between it and your home, which can allow a lot of water infiltration in a short time, so act quickly in this case.
3. Moisture Infiltration
If you start to notice more moisture in your basement or between your walls, this could be because your siding has become compromised and is allowing the water in. Even if you cannot see any siding damage yourself, you should have a siding contractor come take a look and see where it is coming in. There may be some rotten wood in a hidden spot, or some of the siding may have been chewed on its underside by termites. New siding may be necessary in order to keep the water out.
If you do notice any rotten siding, gaps between boards, or moisture filtration, do not ignore it. Wooden siding does not last forever, and these are signs that its time might be up. A siding contractor can further evaluate the situation.