** Updated - New Crop Withering Times **
Crops start to wither when their growth time has doubled.

There is a 50% cushion time during which your crops will start to wither.

Crops wither randomly, not in the order that you planted them.

Example:

Blueberries take 4 hours to fully bloom.

Blueberries will start to wither 4 hours after full bloom.

Blueberries will keep withering, bit by bit, for the next 2 hours (2 is 50% of 4).

As you can see from the chart above (see Harvest Profit Margins), blueberries can be very profitable... that is, IF you can harvest and replant them every 4 hours. Unfortunately, they also die quickly and can be a profit buster if you can't get to your computer to check on them.

Plant crops that you don't have to babysit as much if you cannot get to the computer every 4 hours or so.