Guess it depends on how we define a "food" plot.
In a more traditional sense, will be less than 1% in cultivated "planted" plots.
But have about 10% of fresh clear-cuts (cut this past winter) that is now hundreds of acres of young ragweed --- and the deer seem to prefer that over anything I've ever purposefully "planted". That will become hundreds of acres of young greenbrier, blackberry, and honeysuckle this winter. Also have some mowed hay fields that always have a good bit of clover, yet don't look at them the same as the "totally for wildlife" FOOD plots. Then there are some fallow fields that get rotationally mowed, some parts every 2 or 3 years, some parts annually.
All in all, the deer where I'm doing most of my TN hunting seem to be very healthy, even with few traditional "food" plots and virtually no agricultural crops within 5 miles. I attribute this localized herd health to our having been "hammering" the does for many years, and keeping the deer population very healthily below what the habitat COULD support in the short term. As much of this habitat (lots of both recent select and clear cut) matures, the healthy carrying capacity will be significantly reduced. May need to install more "food" plots in the future.
In the meantime, my view is that cultivated "food" plots are highly over-rated and generally a poor use of my time and money (compared to just habitat manipulation in general). In terms of "pay back" for my time and money, nothing has compared to a brush mower and a chain saw for me.