Had to change the guitar's strings last night. I've spoken to this before, but the gist of it is that for me, I can get about five or six months from a good set of classical strings–I use Aquila Alabastro Normal Tension, what they call "Nylgut™," the trebles of which are white nylon and made to replicate the warm and woody sound of gut strings. (Gut strings sound great, but they cost five times as much as nylon, they don't last long, and they go out of tune frequently. Plus you have dead sheep on your axe ...)
Nylon guitar strings are classed by tension and material; steel strings go by gauge and material, and some are coated with stuff to make them last longer and play smoother. Some are wound flat, for a smoother slide. A nylon-string guitar is something of misnomer, since the three bass strings are metal wound around a core, brass or bronze winding, sometimes silver.
String wiki here.
Guitarists tend to look for strings in the way that the Templars looked for the Holy Grail; you seek the perfect ones, but you seldom find them, unless you are lucky. I've tried fifteen brands over the years, and these are the best I've come across. They make my guitar sound better, and I like the tone. Plus I need all the help I can get.
I keep my guitar clean, wiping the fretboard and strings frequently, and so they don't get too cruddy. When the trebles get stretched past a point, they start to sound dull, and you spend more time tuning than playing, which are signs it's time to swap 'em out.
It doesn't take that long to install a new set. I have a winder somewhere, but I usually don't bother with it. The biggest drawback to new ones, especially those on classical guitar, is that it takes a while for them to settle in, they keep stretching and going flat, so you expect to spend a few days tuning more frequently. (One trick is to tune them sharp and let the guitar rest overnight, and I've tried this, but I haven't noticed it does much good. The strings seem to have a kind of memory, and they go sharp or flat to get back to where you tuned them.)
Mine are spendy, but at eleven bucks a package and six month's life, I can't complain. And for a few months, I'll sound better than I am ...