All the advice you have had is good although it will vary a bit. Personally I wouldn't just play all day when you're a beginner, but break it down into a few shorter sessions. I've been playing a long time and have pretty good callouses but even now after a couple of hours it can hurt a bit!
Ref fingernail length, I much prefer to file my nails with an emery board rather than cut them; I do it every day or every other day to keep them short (fretting hand) and as I fingerpick, keeping them at just the right length on my picking hand every two days with an emery board helps to stop breakages.
As your callouses develop you'll probably find at first that they get hard and may crack a bit; and you may get 'ridges' where you practice the same few chord shapes; whatever you do don't pick at the hard bits!! I also use a light emery board on the callouses occasionally if they are a bit 'ridged' just to make them smooth but only a light rub.
To build up strength in your fingers you can buy a cheap finger stengthener; planet waves varygrip or gripmaster; about £10-12 in UK. Good when your tips are sore and also in the train, bus, at your desk, anywhere. It particularly helps to strenghten the fingers you use less as a beginner i.e. 4th and pinky.
Keep going; no pain, no gain