If you work fast, and you take an effective approach, the quest can be done in an hour or so. However, if you are unlucky, you can spend over 4 hours completing it. This is due to the fact that your plants can die if your farming level is low or if you are just unlucky.  The best way to make sure the quest is over in under an hour is by doing the steps as fast as possible. Do another part of the quest while your plants are growing. Another good way to make sure you do not need to start over is by using plant cures on the plants.