I am a big fan of "baby jail" - a play pen. It allows younger children to be doing stuff with minimal supervision while the parent/s are doing other things. This does not mean NO supervision... if they go quiet, check on them, but it allows you to be doing the dishes or cooking etc without them getting underfoot.
As for the general baby-proofing, the stairs was the big one - our house is kinda upside down so we live upstairs mostly... and our staircase was pretty much impossible to do using a regular gate so we stuck a big plastic table on its side.
Most of the other baby proofing is down to you should do it because you will horribly regret it if you do not and something happens but unsupervised kids will find some way to hurt themselves no matter how much baby-proofing you do. You NEED the supervision.
The other trick is that the kids need to be scared to go into the kitchen or out of the house without permission - Overreact just a little bit the first time or two they do this. This sounds mean but you will not always be watching them and things can go bad very quickly - grabbing the side of a hot frying pan or running into the road. Much better that they simply don't go through those doors without permission. If they ask, take their hand and take them through the doors yourself so that they are rewarded for asking (and so they know that it is the lack of permission that's the problem, not the doorway).
As for the age when you can stop baby-proofing... mostly it will become second nature to use the baby-proofed cabinets etc and you'll never get around to taking them off until they simply stop functioning. When moving into a new home, you'll already know what your kid/s are prone to do so protect from the major risks.
Good luck and best wishes