Change strings when you cant tune well by ear. Dont use a tuner at this questionable stage of, "Do I need to change my strings?"
Simply tune your old "A" string with the tuner, then tune the rest of the strings by ear. If you struggle hearing the strings ring out true, and the sound is kinda muddy, and the tone as you are hearing it kinda wavers up and down (getting overtone vibration decay),change out your strings.
To develop your ear (training), immediately repeat this same process with the new strings. You will definitely hear the string's ring true and clear, and tuning is faster...when you twist the tuning peg up to raise the pitch, you will hear definite clear tone/note. With tuning by ear, tuning in octaves, be sure your guitar is intonated....learn to do this yourself, and check intonation after new strings are installed.
For me, if my guitar keeps going out of tune, I need to probably change strings. Their appearance can also tell you to change out. Clean your strings with a lint free towel that wont scrtach your finish. I use Fast Fret to clean my strings. Hand wash all towels in the sink with baby powder soap and some fabric softener. Fluff dry in the dryer.
Psychologically speaking, buy your strings in bulk, and DONT HESITATE to change them when you feel like it. This is a nice little luxury you give to yourself to enjoy playing MORE, as opposed to not playing cause you dont have a spare set of strings.
I did this with buying picks....when I was younger, I had 4 picks to my name, and could not afford to loose them. So I bought a gross (144 count) of picks, and relaxed knowing I could loose a pick, or give them away. I stopped asking myself if the pick was worn down and needed to get a new one. I throw some what old picks inside the sound hole of my Epi A-12 acoustic that sits out in the living room. I can always get to a pick , and have a maracca.
I keep a few picks in each car's glove box, and one in my pocket at all times...for spontaneous jams!......Buy the 10 pack of strings at Guitar Center, which is cheeper, and I always go in the store and ask them if they have any "string deals" going on. Also keep extra 1st, 2nd, and 3rd strings sealed in a zip bag with a desicate pack in every guitar case, in case of breakage. All my cases have their own string winder, picks, and capo.....a luxury I gave myself over time.
Save your string packages to give to family members so they know exactly what brand of strings to buy you for Chritsmas. Try to teach grandma how to say "D'Addario". Just give her the package & send her to the guitar shop. I get pretty much outfitted with strings for a year every Dec. 25th.
Sorry so lengthy and anal. >> DAN
79' Epiphone Genesis Custom, 89' pre-reissue Les Paul Standard, 90'Strat Plus,
02' Tele (ash), 91' Martin HD-28, Epi A-12 acoustic, Fender Hot Rod Deville 2x12