There's no single static variable to define how long change takes. It depends on what you're changing for starters for if it's even possible, but then there's also who/what is changing it, what steps are being made towards the changes, all sorts of things that can hinder or optimize progress.
Well, of course with that defeatist attitude, you'll never change. Your choice. Your entire personality can be changed if you just will it to.
I think you half-assed want to change. You think you do for the sake of convenience in life, but you somewhat take pride in the fact that you stand up for yourself, even if a little too much. Until you fully accept your flaws as flaws, you won't change. Of course, that's just my observation. I could be wrong. Heck, I don't even know the context you're speaking in.
That doesn't mean standing up for yourself is a flaw. It just means not being able to recognise when you need to step back for the sake of achieving whatever you're not able to achieve currently, is a flaw.
I used to have a similar problem, when I was younger, with my mother. As I grew older, I still had the head-strong attitude and held the same beliefs, but instead of telling her she was a fucking dumb piece of shit cunt and having to breathe before I bashed her in the head with the nearest hard object, I would chuckle at her stupidity, and walk away with a sarcastic 'ok, mother.'
Control is the thing you need to learn, not how to be a pushover. Never be a pushover.