I fertilize when I log in, a second time when I'm getting ready to switch gears between content grinds and then a third time when I log out. Sometimes they'll get more than they benefit from but other times it makes up for the days when I play less than usual and fishmeal is cheap enough that a little waste isn't a concern.

Quote Originally Posted by Besame View Post
If you have no life-fertilize, otherwise, don't bother
If you fertilize 2-3 times while growing (not every day but total), it means the crop should be ready to harvest when you log in instead of needing to wait until it's been that exact 5 days (or however long) from when you planted to harvest. Without fertilizer you'd eventually end up at the point where you're waiting an extra day to harvest because you need to log out an hour before the crops are ready.

If you only occasionally grow crops, it doesn't matter but if you do it regularly it makes a noticeable difference over time.