With no roads, it takes 10 minutes to generate 1 food.
With 1 road, it takes about 8 minutes 32 (or 33) seconds. You save just under a minute and a half.
Additional roads will further reduce the regen time. But each additional road reduces the food regen time less and less. ex. - with 2 roads it will take about 7m 30s ... So with the first road you saved about a minute and a half, whereas the 2nd road only saved you about a minute.
I'm just guessing on 2nd road, because I don't remember exactly what it was.
Maybe someone who has 2 roads can give us the actual amount.