More like 4-12 hours per day depending on the age of the farm (startups take work), the quality of the land and the crop season. More hours during planting and harvesting, and almost nothing during the winter. The number didn't really depend on technology past a certain point of desperation (passed well before recorded history.) For example, the gothic cultures farmed before they borrowed the plow from the Romans. Post-plow they became richer instead of cutting back hours. No different than today.

There were many provisions for crop failure, including crop variety, raising animals, preserving food, fishing and trading with luckier regions. Like today.

Pro-agrarian looking to set the record straight. I'm also in favor of nuclear power!