9/10? Never. Traditional payment for church was 1/10, and lords used rather unpaid workers than direct payment. The forced work was not too much, so they could keep their own farms and fields and do enough work for themselves. They did not like it of course, but overall taxes including the forced work were far lower than we have today.
How much they ate - we do not know exactly, but there are some sources from 16th century; not very exact, but it gives the idea. So the workers on farm (not the farmer's family itself, but hired workers) shoud get three times a day the cooking (which means meals from boiled peas, pearl barley etc. - very common type of meal in Bohemia), a piece of meat (3 times a day!) and a piece of bread. The same was common for mill workers later, it was probably generally accepted practice.
We also know something about priest's meal, which is interesting, but of course different from peasants. In 12th century priests from Vysehrad (in Prague) had their own fields for growing wheat. Apart from that each of them recieved weekly 12 chicken, 6 pigs (small ones .. don't know correct translation, maybe "piglets"?) and 1 young cow. If the cow was not available in winter, they recieved two 3 years old pigs instead.