Guitar Stringing How To

How can I tell when I need new guitar strings?
I’m a novice guitar player and I have an acoustic guitar. I’m just wondering how you can tell when you need new strings because mine seem a little worn right now but I’m not sure if it’s actually time to change them. I’ve had these strings for about 2 and a half to three months. Please give me some tips on how i can tell when to change the strings. Thanks.
-if they are hard to keep in tune
-the lower strings aren’t as clear, they sound more like a thud
-they’re dirty
that’s about all I know, but I also play bass. I know if you clean them regularly and wash your hands before playing they have more longevity.
Stringing An Acoustic Guitar