I've had random stomach problems (mostly unexplained D, pain and bloating) my whole life, but it was been most severe (i.e. debilitating) starting about three and a half years ago. although I'm quite certain I've had IBS my whole life, being able to recall various episodes when it flared for seemingly no reason, I wasn't diagnosed with IBS-D until about 2 years ago. however even before being diagnosed I was so sure I had IBS that I started H's diet and lifestyle changes probably about 6 months before that. so I've been on the diet et al for about two and a half years and I've been stable for about a year now. so yeah, it took me a year and a half to stabilize, though I did see some pretty immediate improvements (basically that I was able to stop having major D attacks after every meal! I still got D, but it was much more under control within a few weeks - i.e. 1-2 attacks a day vs. 5 or more! - while pain and bloating took much, much longer to deal with.)
I should say that yes, I did eat somewhat restrictively (but still healthfully) for that year and a half while I was stabilizing. LOTS of SF at each meal, and incorporating IF fruits and veggies back in very safely and very slowly. I was (and still am) extremely careful and diligent about everything that goes into my mouth (though being stable now I can be a bit more relaxed about it, plus most of it comes second nature now!) yes it can seem a little boring at first but a) you get used to it, and b) it's better than having 5-10 D attacks a day!
I'm at a point now, thankfully, where I don't have to think about my IBS all the time and I can tolerate a lot more fat and IF in my diet than I used to be able to, which means I can eat a much more varied diet (yay!) I've been able to add back in lots of fruits and veggies (which is fabulous) though I still stick to mostly SF grains, like basmati rice, white breads, pasta and oatmeal.
this is my general routine/what helped me stabilize:
following the diet strictly (no trigger foods, low fat, always SF first etc. - and no cheating, ever.)
acacia (1-3 tbsp a day depending on the day! most often 1 or 2tbsp)
immodium as needed (not very often)
accepting that I have IBS and not stressing about it is probably the most important thing I had to do to stabilize, and really it took me a year and half to really completely do that. after all the diet rules became second nature and the lifestyle changes were part of my life, it was really a matter of starting to think about my IBS as something that's part of me rather than something that's happening to me (if that makes sense...) once I did that I was well on my way to become stable. I also had to stop being embarassed about it and just tell friends about it without stressing, which means now that I have friends to support me and who understand why I may not be able to order anything on the menu, which is a huge stress off for me. I really think that acceptance and keeping stress (in general yes but particularly stress about IBS) is just so important.
other than that, just staying healthy - eating well/regularly, lots of water, as much sleep as I can fit in, exercise almost every day (really important for me) etc.
did I mention no cheating?! I can honestly say that I haven't had a single trigger food since starting the diet. (well ok once this year, after I'd stabilized, staying up all night madly trying to finish an essay for class I had ONE cup of caffeinated TEA... and dearly paid the price but that's it! in the whole 2.5 years) really there are so many substitutes that I hardly ever want to cheat anyway! plus I know that the pain of an attack will be worse than the enjoyment I'd get from that pizza, so it's really never worth it in my mind.