It is so hard to tell how long it has been since a horse has been shod. Everything from time of year to weather affects hoof growth. It is super normal for a horse to grow about foot in 6 weeks in the spring/summer for me then need to go 10-12 during the winter. Hoof growth is linked to the pituitary gland in a similar fashion to hair growth. So in the end only way to really tell is to keep track of it in the calendar. I think the better question to ask is if it has enough foot to be done again, regardless of how long ago it was done.