I think the answer is on the third day.
I've never sprouted lentils, so I googled it. Quite a few responses. Here's an excerpt from one website and the link to the full website. I left out the steps that you likely already practice if you've been sprouting.
Let sit in water overnight(12 hours).
4The next day drain off water.
5Rinse and drain once or twice daily and keep out of sunlight.
6On the third day (some people wait till the 4th) you will have sprouts with about a 1/4-1/2" tail on them. www.food.com/recipe/how-to-sprout-lentils-