I rarely change my strings, other than to try another gauge, unless ( as Madiac said ) they sound 'dead' .
Instead , I carefully wipe them ( not the fretboard ! ) down after each playing with a lint free cloth which has been slightly impregnated with WD40 - and that way they look 'new' for months - whilst retaining that balance of clear sounding yet mellow that I like. They also feel lovely and slinky to play on that way.
I don't like the tone or feel of new ( sets of ) strings - they don't seem right until they have stretched in, 'relaxed' and lost a bit of that brash edge.
Do remember that it is rather 'style dependent' - and my fave strings are Thomastik Flatwounds . If I was a 'Rock God' - ( as many are on this forum ) , or a Country / Funk player - I would probably be changing them often to achieve just the opposite effect.