It's generally done when the plants begin blooming.
Frankly, I wouldn't recommend it, though. You are more likely to damage your plants than increase fruit set. The hormones used to increase fruit set are the same ones that have been used as herbicides for decades. The dosage is critical, and not that easy to control.
Commercial growers use hormones to overcome issues with specific weather conditions to maintain consistent yields. But, there isn't much to be gained from their use by a home gardener or small producer.
