The timers in Virtual Villagers, including the crop timer, rely on a stable and accurate system clock in order to operate properly. The crop timer should vary with game speed, but if your crop timer is consistently more than 400 minutes, it would be a good idea to check to make sure that your device's clock is set to the correct date and time. We have never seen problems with the crop timer except when the device's clock is being changed, either intentionally or inadvertently.