It depends on if you're on the 6 or 12 step and if you're instructed to do 3 or 7 days per step. For my son's severe ige allergy we got the 6 step 3 days each and started at 13 months passed it first try in 3 weeks.
We did 6 steps and 3 days . Took a year to complete due to having to stop and restart a few times .
My daughter did the 12 step one and was completely fine apart from one stop start towards the end. Did it in 4 months - good luck, I know it’s stressful x
I believe each part of each step should take at least 3 days but people take it slower than this too