I’m pretty much in the same camp as Jake Young above: my main guitar gets new strings about once a month, and my other guitars every 2-3 months on average. Granted, which guitar is my “main” can change on a monthly basis, too, heh.
To Jak Angelescu: three of my guitars have floating Floyd Rose-style trem systems. I remember getting my first floating Floyd back in like 1989, and had serious headaches with them until I changed the strings on it about 5-6 times. My advice is to definitely only change one string at a time to keep the bridge fairly near it’s usual position. Stretching each new string at least three times, tuning it, stretching, tuning … then change the next string. If your bridge floats (can dive down as well as pull up), I’ve found that after all the new strings are on it’s still usually flat in tuning across all the strings. As you bring each string up to tuning pitch, by the time you do string #6 then string #1 is off again. One trick is to start on the low E and tune it a bit higher than its normal pitch (if standard tuning, then just short of halfway between E and F in tuning). Tune the A high, just like the low E. Tune the D and G strings a little higher, but less than the A and low E (about half as high). Tune the B and high E to normal pitch. This makes them come into tune much quicker across all the strings. Maybe that can help. Cheers!