... on what it is there for.
In my world there are various versions of "WiFi Routers" with a number of functions, some of which overlap.
Until a few years ago my one and only "WiFi Router" was there to connect my computers to the internet via ADSL. The laptops and phones - which all understood WiFi - and my printer would connnect directly to the router's SSID, my network switch - and through it all computers connected via Ethernet - would be connected to one of the Ethernet ports on the WiFi router. Thus they all gained connectivity to the world, but the WiFi devices also gained connectivity to the machines on the wired network.
Here is where an important distinction comes in: A wired home network today would typically run at 100Mbit/s or even 1Gbit/s. An ADSL line in my neighbourhood can do 4Mbits/s - that's all my Telco can sell me. Fiber is not arund the corner, LTE (at up to 150 Mbits/s if you are lucky) is still quite q bit more expensive if you need volume.
The reason I am explaining this is that for internet connectivity it doesn't matter too much if my ADSL and WiFi router supports all the highest WiFi speeds - the ADSL can't keep up anyway. For connectivity between my computers, however, it does make quite a difference.
The next consideration for how often to replace my router is very compelling: Whenever it is destroyed by lightning - in this neck of the woods that was about every three months recently. Needless to say, I now don't by the most advanced model but rather a cheapie on which I can get a good discount if I buy a sixpack.
In the mid term I am hoping for fiber or I may bite the bullet and go LTE (and some day 5G) for all my connectivity. At the moment I have a little router (or use the one built into my smartphone) as a backup solution when needed. Disadvantage: It is either impossible or complicated to connect the wired machines to the internet with these or the WiFi enabled devices can get to the internet but can't see each other. These solutions also support a smaller number of connections than the "real" router. And reconfiguring the WiFi based printer is a drag ...
But in general, there are two things you can do to stretch the lifespan of your current investment "in the router space." - One is to upgrade the firmware from time to time - especially useful if you are sensitive to security issues and want related bugs fixed. The other is the occasional reboot. Many of us probably get the odd reboot here or there for free - power outages just happen from time to time or, like me, you need to unbox the next router after a lightning strike and then you start at square 1 anyway. Why rebooting helps many an issue? Memory leaks and garbage collection. Back when I learned programming (on "mainframes" with 128 KB of memory and half a MIP) you debugged the memory behaviour of your programs. Nowadays you are encouraged to rely on garbage collection in your environment - which may run smoothly or not, depending on implementation. The software may also collect memory segments it doesn't really need anymore, so as a result you will eventually run out, which makes garbage collection more frequent and your system (in this case your router) still runs, nominally speaking, but it actually just limps along. A reboot will free all the spare memory and your router will appear like new. (Works with PCs on Windows, too - older versions would get their reboots automatically; so does version 10, but in between you had to watch the task manager to see how memory was "wasting away" -
So, in order to have some semblance of "business continuity" people here run things on batteries and inverters, which deprive all these systems of their periodic power failures and corresponding reboots. So do yourself a favour - like MS has a "patch day" treat yourself to a "reboot day" about every four weeks and include all your devices.
Lastly, if you hear about a technology upgrade that you find useful you may want to take that as an incentive to upgrade the router - but then also check, which of your WiFi capable devices can and should be upgraded as well.