How long any given set of strings will last depends on how much you play and how sweaty your hands are. Cleaning the strings after you play will delay the corrosion. Elixir strings will generally give you longer life, but some people don’t like the tone while others don’t like the feel (they have a thin coating). They are also more expensive. I use them, although I prefer phosphor bronze strings. I use Elixers simply because they last longer, sound & feel ok to me & I’m lazy.
The better your guitar sounds, the more you will play it. The more you play it the happier you will be.
When I change the strings, I change them all, one string at a time. The only time I have ever oiled the fingerboard is after a prolonged period of non playing. If I am playing it, it is getting plenty of oil from my fingers.
I might as well pass on the most useful guitar tip I have ever come across; make playing your guitar convenient and comfortable! Have your guitar out and on a stand, not in the case under your bed. Have it right next to a comfortable chair with a good music stand in front of it. Have everything you need within reach (picks, tuner, books etc).
The more convenient and comfortable it is to play the guitar, the more you will play it. The more you play it, the happier you will be.
Here’s a couple of tidbits to add to all the good info here. There’s a product called, IIRC, Gorilla Snot. You apply it to the strings, and it allegedly reduces friction and reduces that skreeeek you get when you change chords.
Before there was Gorilla Snot, a few 60s era guitarists would rub a bar of soap onto a new set of strings. It allegedly did what GS does, plus giving a slightly deadened tone in that time of screaming distortion.
Ethelbert makes an excellent point. One of the most effective things you can do to prolong string life is wipe them down well when you’re through playing. True dat.