Three days of fishing or two days of fishing

This is a traditional way of fishing. Specifically, "fishing for three days" means going out to sea every day for three days to get a catch.

"Two days of drying nets" means that in between fishing trips, two days are spent repairing and drying the nets in preparation for another fishing trip. This idiom can also be derived as an attitude towards study and work. It implies the need for a balanced and regulated state of study or work, where one must work and study hard while resting and relaxing appropriately to maintain an optimal working and learning state.