If my experience is anything to go by mindfulness meditation makes subtle and very noticeable changes (for the better) and (also in my experience) fast.
I began mindfulness because of ill health, I just kind of got drawn to it, very much. I suppose originally I hoped for the goal of feeling relaxed, peaceful and for my body to begin to heal. I did get some of those things happening, but the first time around I didn't 'click' with 'just being' and 'accepting' throughout the day and during meditation and I was still in a very self judging/analysing stage.
I got very unwell again (relapsed) and also got pneumonia so everything went out the window, though I have to say that my practice had been very hit and miss due to me not 'getting' it. Like the body scan (for example) I used to find it a chore and be so reluctant to do it and 45 minutes seemed so long, now....the difference is unbelievable, I love the body scan and can't wait to do it and am so disappointed when it ends.
I know the thing that has made a difference for me this time around is that I remember to stop every so often during the day and connect with my breath and body or simply look at what nature has to offer and have mini meditations. I meditate when I walk about and when I eat and so on.
I still have distractive/entertaining/tantalising thoughts that tempt me and sometimes I succumb for a while, but then I become aware and I let it go. I name it if it's particularly strong and that seems to help. For the most part, though, my mind is calm and that is an awesome feeling, believe me.
I think what I'm trying to say, in a rather disjointed way, is that meditation/mindfulness is now absolutely spot on for me and I understand it much better, I am ready for it now. Years ago maybe the time wasn't quite right but I'm so glad that I discovered it so that I could develop it when I was ready.
I can't imagine any other way to live now