This is probably extremely variable depending on the specific crops you are talking about. I tried to gather information from various sources. I looked mostly at studies of subsistence farmers; I propose that the labor per unit area would be similar without any mechanized methods. Yield may be higher based on other factors like increased availability of scientific information and fertilizer. Yield is, of course, also heavily dependent on crop type.
A study in Bang Chan Thailand found an average of 3.4 man-years per 13.6 acres, or 1.6 hectares per person.
A study in Madhya Pradesh found 2268 farm laborers managing 7811 acres, or 1.4 hectares per person.
A study in the Kandyan Gardens of Sri Lanka found 30 farms with 2.54 adult equivalent units of labor tending 1 hectare per holding, or 0.4 hectares per person. However, these are high intensity 'food forests' where coffee, black pepper, tea, and spices are grown in conjunction with food products like jackfruit, coconut, and plantain.
A small sample size study in Kakamega, Kenya found farmers tended on average 1.2 hectares per. However, it was unclear if contributions of other family members were being measured properly. Also, like the Kandyan Gradens, this isn't strictly subsistence as only ~60% of the planted areas were food crops (maize).
Thats all I could find for now, I'll update if I can see some more.
The evidence was remarkably consistent between studies. I looked at about 25 studies, and these were the only four I could get labor per unit area numbers for, so it is as random a sample as I could provide. It looks like the labor requirements are such that one adult can tend 1 to 1.5 hectares using subsistence methods, and less if a more intensive food forest approach is used.