I check their food at bedtime. If I refreshed it within the previous couple of hours, and it looks like there's enough (seat of the pants assessment), I leave it be. If it doesn't look like enough - and they do snack during the night - I switch it out with a fresh pan full.
The food, by the way, stays in the veranda with their water. I used to remove their food overnight, but nothing else has expressed interest in their food, so now I leave it with them, hoping they'll ingest a few more calories and some more calcium. They make a bunch of eggs most of the year.
Right now a number of them are molting so we're getting about three eggs a day. In a couple of months I expect that to increase.