We always planned one Wishes and one Illuminations night. I figured if we made it fine, if not, there would be a next time. I usually kept those evenings towards the back 1/2 of the trip because by then, my DGD would have gotten a little used to the excitement so all her adrenaline had evened out, and she was taking an afternoon nap, along with everyone else, i might add! LOL (but me, I hang at the pool with a cocktail and a book)
I would build them in, planning on a later arrival and perhaps dinner in the park to give a break.