Do you mean would using straight milk take longer to show up than hidden dairy? If so, then I would think that with a shot of plain old milk any reaction would be faster, not slower. I think that is the other reason for using milk rather than hidden or trace dairy -- if you go with hidden and he reacts or has say tummy problems or funny poop or a rash and it's a day or two later, are you really sure that it was the hidden dairy or could it be totally something else? There is always some room for second guessing with any food trial so going the straight milk route removes as much of that second guessing as possible (but still leaves some I guess!)
What they do in the hospital at 12 months in my area is a syringe with 1ml, then one hour later 2ml, then an hour later 3ml and so on until they hit 5ml with no reaction (and I have to say all I could think of was "um, and what about his naps?!!") DS was booked in for it ages before we knew we were moving abroad and as we moved the day after he turned 1 we had to cancel; they would not do it before - one of those silly cases where the computer needs him to be more than 12 months for them to be allowed to book it in, so they could not do it at 11 months and 3weeks old. So we were left on our own to do it.
I will be honest and say the 5ml was just because I was lazy!!!! I gave him 1ml at first and it seemed so pointless when as I said I was pretty sure he'd had butter a few days before. So I just gave him the other 4mls a few minutes later. I was totally at the end of my rope with the milk thing (I know everyone is! It is so hard to read labels in another language when you are struggling to figure out what the food is in the first place!)
I just kept my eye on him for crankiness, nappy problems, rashes, spit up, and so on, and then did another 5mls the next day (or possibly waited 2 days) and when there were no issues I just went ahead with it. The first while we were still careful about things like not overloading him on dairy as I think that as with any "new" food too much can be too much, iykwim. A very short time after he was newly onto milk whatever I'd made for dinner was with a heavy cream sauce and I remember having to explain my logic to DH about why I was not letting DS have any...and it was just because I feel that a heavy cream sauce can be hard on MY tummy and I have no milk issues, so for him, who was just getting used to milk/dairy it would be a thick and heavy thing to digest. Kwim? We kept things quite simple at first, just a wee pot of yogurt or slice of cheese each day and then after a few weeks just went on with things as if he were any other toddler and had never had a milk problem in the past, kwim? I admit that I probably got off easier than most in this case so I am thankful for that!!!