Captain M is very correct in what he says about this being your choice and it's your time scale, but what Green Mango said about your NPDh picking up on subtle changes to behaviour, is also very true. This is a violent man who thinks nothing of using rape to control you, and that is whilst you are conforming and staying with him!
You mentioned that you think this could take a year to actually come to your leaving the relationship. A year is quite a long time for someone to notice changes in patterns of behaviour etc. If you are going to wait up to a year before leaving, then as Green Mango said, you are going to have your work cut-out to be as vigilant as you will need to be! I don't know if I could keep up a convincing act for so long! You are going to need to cover your tracks, act as if nothing has changed, talk about the future when he wants to, as convincingly as you used to do etc etc
I am very worried about what further damage he could do to you and your children over the course of 12 months. Given what he has put you through physically & emotionally, he is clearly a very dangerous person. I would hate to think what he would be capable of doing, if he got wind of you planning on leaving him. Narcissists do not take being left very well. Narcissists are generally the ones who do the leaving.
All you can do is take all of this into consideration when setting your time scale, and try to make provisions in case things turn sour prematurely (more sour).
Do you have a member of your side of the family or a close friend that you could trust with your life? Could you make them aware, and could they be prepared to take you and the children if you had to make a quick exit?
Perhaps have a code-word or phrase you could text them, if something changed and you needed help quick. Just to protect yourselves. You could text 'How is your friend doing after her fall'. That would be something you could save on your phone as a draft, and then send in an emergency and your friend would know what you meant. That way, even if your h came across your phone, he would not suspect anything.
Could you perhaps begin to store some things very gradually at your friend's house? Perhaps bit by bit you could get things like your driving license, photos, birth certificates, your passports, very important things of sentimental value, and anything other that you would have to have with you, and anything that he may withhold from you out of spite, should he find out your plans. Obviously if he tends to take care of household documents, then he may notice some of the above documents missing, so perhaps photocopying things might be a compromise, or getting certified copies of things made and stored. I've heard people on here mentioning that it's important to get hold of the marriage certificate, if you are hoping to file for divorce at some point.
Perhaps a good idea if you did have somewhere to store bits and pieces, would be to let your h think you are having a clear out. You could genuinely put some old things in bags to take to a charity, but then put some clothes for you and the kids in another bag, and take that to your safe place. Even if it were just enough clothes to get you all by for a few days, until you were in a position to get back into the house for the rest of your stuff, once you'd left.
I don't want to appear over the top or that I'm trying to scare you, and it may be that you never need the provisions you will make, but at least if you prepare for the worst, you have peace of mind that you've covered yourself.
I'm sure you will get loads of advice from people on here as to what things to watch out for, and I think the suggestion of the legal board on here is great advice.
I think you are brave and inspiring, and your kids will be very proud of their mother, when they come to understand one day.