It is clear as mud and does take a long time to figure out for sure. It was about four months for our DS and I thought that was pretty quick in the grand scheme of things. I have heard 6 weeks for casein and 6 months for gluten if you want to be sure. Not sure about soy and eggs. I will say we had immediate improvement with cutting dairy but then we subbed with rice milk and DS is the only person I know who has major major issues with sunflower oil which is in rice milk and just about everything else. We also did an allergy panel on him with about 20 foods. It turned out that he was allergic to nuts, tomatos, and soy. I would have not guessed it until I knew. Despite having food allergies myself, somehow I failed to notice how red and itchy his face would get when he ate tomato products. Anyhow, it is a process, but if and when it works, it is sooooooo worth it. It feels like torture to not be able to eat the foods you normally eat, but in time you really do start focusing on what you can eat and it doesn't seem so bad. DS has a really great varied diet now even though he has the four actual food allergies and probably 20 more sensitivities.
Personally, I thought pulling everything at once and then adding back was easiest. Because for instance if you pull soy one week but you're still on corn, you may not notice that soy is actually an issue because corn is still bugging you.
A book we really liked that had four lists in it starting from least restrictive to most restrictive was called Feast Without Yeast by Bruce Semon. Book is on Amazon. It gave us good ideas of what to pull first. Something like 80% improve with the first list and so on. Lots of recipes and also lists of what you can have. DS does not have ADHD, but has major sensory issues and probably ADD as does DH. The book is really good for those too.
Good luck. It is a long road. I recall sobbing in the grocery store the first time I went looking at labels because I didn't know what we would eat. Looking back it is almost funny because we've found sooooooo many substitutions. If you want me to pm you, I can send you a list of some meals the whole family eats, certain brands that are good, etc. DS does not eat gluten, casein, soy, sunflower, tomatos, nuts, oranges, vinegar, malt, anything with added yeast, maple syrup, corn, chocolate, eggs, apples, bananas, grapes, avocado, any artificial colors/flavors, and probably some other things I'm forgetting.