I think it is relative to the violinist/violist/cellist. I think that when a person wants or even needs to change strings has to do with what makes them happy, combined with what they can do financially, or need to do it.

I change mine when I am just tired of hearing them and want a change. If they are still good, I have backups. I know that if I am just tired of the tone from a set, I need to change them, even of they are still good, because it bothers me and I concentrate too much on wanting a change. Sometimes they wear out before I tire of them. Sometimes, I just want a change. I want to try for a different overall sound.

I don’t think there is a set rule, unless you only change when needed. Even then, it depends on the instrumentalist. How often it is played, is it played hard with aggressive bowing, or more subdued slow peaceful graceful songs. I think that if you change only when the strings wear out, all of that plays a part.

So, I really do not think there is a definitive answer, it is relative to so many factors for each person. Much like setups.  

@ELCBK and others reading this topic

I have been lucky in that most of the strings I have tried have settled in fairly quickly. Some I liked for a couple weeks, probably because they were different, and then not so much because I notice something that I was not getting from them, some tone or something. Not warm enough, etc. Most of the time I am happy with them, I just need a change, like moving furniture to make a change.  A lot of times, I will put them on again, when I want a change again. I will do that until they are no longer good. 

There is one brand that I absolutely cannot stand and could never get used to any string, C, G, D, or A or G, D, A, E. Depends on which instrument, but I don't like them on any instrument. I am not saying the brand because it is not that the strings are bad, I just don't like the sound, which, again, is personal preference. 

I don't say a brand name if it is just that I, personally, do not like the sound.

These are what I like on my warm violin:

Fiddlerman - I love these the best. They seem to fit this violin. Good thing, because they are the least expensive.

Evah Pirazzi Gold - second best and are what is on it now. I broke a Fiddlermam string, my fault, not a manufacturing issue. I had a set of these and put them on. I prefer a complete set of one brand/model. Unfortunately, I really love these, too. The E is lovely, but the Fiddlerman ones are better on this violin. If I still have this violin when these wear out, the Fiddlerman strings will be put on again. I bought another set.

Obligatos - Because they behave like for me gut, I do have issues with the response. I am not experienced with playing my violin, so I have a hard time with these. I was told the that they were gut, I figured that person was right since I had just started violin a few weeks before I got them. I have since found out, tonight, that they are synthetic made to act like gut, but I guess that would be relative to the instrumentalist, too. I tried them a few years ago, when I started out. I had to remove them, and still have them. The response is too slow and require better bowing capabilities than I have.

On my brighter Concert Deluxe I have had

Kaplan Ammo - nice on that violin - I believe that is what it came with, and has been the best suited on it.

It has Fiddlerman now. The Kaplans wore out and I had Fiddlerman strings, other than the Obligatos that I removed from the warmer violin. 

I like the Fiddlerman Strings on it, but it is not as bright and fiddle sounding, which is what I play on that violin. When I can, I will get Kaplan Ammos, or I will try a lesser expensive set that is bright. Maybe Tonicas. I had them on my first violin. Really gave life to that inexpensive first violin. But, that violin sounded much different than this beautiful Concert Deluxe violin, so I am not sure they will give the Concert Deluxe the "respect" it deserves. If they are a lot less expensive, it is worth the try, rather than the Kaplan Ammos. The Kaplan Ammos are great on it.