Time to change strings

I thought it’s because you’re allergic to changing strings?

Alan2007 yes spot on hence the existence of coated strings. Coated strings are not universally liked and come at some expense but need changing less frequently which makes them more affordable. Old school practice was to wipe the strings down after playing and to use a string cleaner such as Fast Fret. The strings least prone to collecting muck and gunge are of course flat wound aka tape wound. There is a halfway house between round wound and tape wound strings ie ground wound. These are round wound which are slightly flatted off to give them a slicker feel. Smoother strings won’t pick up so much gunge will be less prone to squeaking but won’t have the brilliance of true round wound strings. It’s all compromise, apart from it’s always a good idea to wipe strings down!

I am not sure I have experienced that. Coated strings (e.g. Elixir Nanowebs, D'Addario XS and XT, other brands are available) tend to squeak when first put on, but in my experience lose that quickly. I just tried my first set of D'Addario XS, and no squeak. Coated strings do last longer, but they are more expensive. I do suggest you try a set once, to see if that economy works for you, you like the feel, the sound, and so on. 

The point when strings start to "sound dull" can be a bit subjective, especially if you are only listening to your guitar. It might be the boiling frog apologue, i.e. you never notice because it happens gradually. I don't think I have a sophisticated ear for these things, which is why I tend to use coated strings, as I am by definition OK for a bit longer. Sometimes you will see coated strings looking "frayed" at the point where you pluck them - a good sign they need changing. 

If one has a highly developed ear for this, you will change strings more frequently. Martin Simpson, for example, puts new strings on before each performance. It helps if you are sponsored by a string company - keeps the costs down 

Peter 

I like changing strings, it is like getting a new guitar, well nearly. Just changed mine on the Martin D10 and it sounds great.
In the gigging days I was changing strings once or even twice a week so I got quite adept at doing it. Easy on a Strat or Tele.
Acoustic is different I find and it takes me longer.
Hey ho, there's always a challenge.

My Tanglewood had the same strings on for almost 10 years before I finally treated him to a new set

I've never used a string winder. I lock the string on the M/head post, I think they call it the luthiers knot. You only need a couple of turns and you are there. Learnt this in the late 70s and relied on this to this day.
Never needed locking tuners and so long as the nut and intonation are good I've never really had tuning problems.