Flax takes eight hour to be ready for harvest. It is not advisable to plant all 300 Flax at the same time because it would mean you’ll need 300 Energy to harvest all of them. Batches of 30 is recommended but if you’re in a hurry then you can plant 60 at a time but be sure you have lots of Food to convert into Energy.
Harvesting your neighbor’s Flax doesn’t count for the Become Master Farmer II goal. Neighbors who harvest your Flax don’t count as well. But neighbors watering your crops do count as a harvest for the goal. Completing the Become Master Farmer II goal gives you 400 XP, 100 Food, and 5 Horseshoes.
Flax farming in the Become a Master Farmer II goal is just to prepare you for the third part of the goal, which would require you to plant and harvest 1,500 Peanuts.