Every horse is different on the length of time it takes to overcome things, some never do. For safety reasons I use a stock whip extended out as far as I can reach and rub it up and down their back legs this keeps me at a safer distance if they kick out instead of being directly in the line of fire. As they get accustomed to the whip I move closer. I will also use a lead rope to gently toss at their back feet to help them overcome the sudden movements at something at their back end, some it may only take a few times before they settle down others it's an ongoing process for a long time. I will tell you I personally will never trust a horse 100% that has a history of kicking for any reason whether it's a fear issue or a dominance issue.
We have 4 horses now, 1 I trust to never kick, 1 I trust to not kick as long as another horse isn't anywhere near her back end, another I don't trust any farther than I could throw her she has tried to cow kick me and strike out with her front leg at me which btw was immediately dealt with and she hasn't tried it again but I'm not going to say she won't ever try it again. With a horse it's the total package, one that kicks for whatever reason will give warnings in other ways kicking is usually a last effort to free themselves from a situation they are afraid of or simply don't like. If she lacks confidence with people, spend lots of time with her on the ground grooming her letting her get used to you being right there with your hands on her but don't fall into a set routine all the time, change it up from time to time this helps her to get over the anticipation of you doing the same thing and thinking it's ok to not kick now but something different she says to herself "this is different I don't like it so I'm going to make it go away"
I'm sure others will have more advice, this is from my experience and what has worked for me hope it helps some.
Courage is being scared to death but saddling up anyway~~John Wayne