Hi A.,
I'd just like to share my experience--hope it helps.
At about 1-2 mos, it became clear that my son had a serious reaction to something in my diet--he'd scream and scream and the ped was no help: "all babies cry", so I didn't do much at first. But the crying was so obviously because of gas/digestion pain and he was in so much pain, that I tried to track my diet and just couldn't figure it out. I'd eliminate things here and there and it just didn't help. We massaged him, pumped his legs etc. and it din't help, either.
To make a long story short, in my son's case it was all vegetables. Broccoli, cauliflower, peppers, onions and garlic were the worst, but it was all veggies. It was pretty gross, but for several months I was basically on the Atkins diet, plus potatoes and whole grains. I can't tell you how many nights I had mushroom cheeseburgers with tater tots!
But, the difference was amazing!! He went from non-stop screaming in pain for HOURS(while his dad and I held him and cried ourselves because it was so hard to watch) to the happiest baby ever! He just smiles and gurgles and is a delight! So, it's worth it!
At 7 mos, I can now eat salads and little bits of garlic and onions(like in pasta sauce), but no peppers still, and I am too afraid to even try broccoli and cauliflower.
Also, he's started on solids and we are havimg LOTS and LOTS of trouble with constipation. My mom is an RN and says that he just has a sensitive GI, and we are going to have to be very vigilant with him for several years.
It really only makes sense to me in terms of evolution--for the last several million years, our ancestors were primarily hunters, and so our babies are adapted to meat, much less so to a variety of plant foods--which hadn't been domesticated yet and so just weren't available.
My advice is to just narrow down your own diet to 2 or 3 items--meat and a starch like potatoes and see what happens. And give it several days after you reintroduce a food and do it one food at a time. It's not a fun way to eat, but it is sooooooo worth it. I wish I'd done it earlier and saved my son so much agony.
Oh, and the answer to your initial question, how long for a reaction? In my son's case, 18 hrs, almost exactly, from the time something passed my lips to his screaming.
This is a fixable problem--just eliminate everything and reintroduce one food at a time, only 2-3 per week. It's not a fun way to eat, but it's a much better way to be a mom, rather than crying as your child screams in agony for hours on end.
Good luck!