I don't think there is a specific stage to harden off plants, you could even germinate them outside from day 1! I do it one week before transplant date. I Have a large variety of transplant sizes since I'm not a business with a controlled climate, and they go in weather and time permitting.

My major consideration is that the plants are tender and can't afford an attack by anything when putting them out. They are more susceptible to slowing down due to pests, like caterpillars or leaf miners. So I just work back from my plant out date which itself is based on the weather forecast, and subtract a week. If I get to plant out very early, I may barely harden off the plant if the Sun is low, but this depends on the weather profile in your region, milder climates are more amenable.

Good luck!

PC

Good advice about the "when". Putting a breeze on seedlings tends to strengthen their stems. This is well established. To the extent you'd rather the breeze outside not knock them over, that can be helpful. My understanding is that you don't need a fan that makes the seedlings lean over, just air motion that causes the stems to move slightly. I suspect a shake-table that just vibrates the seedlings will do the job too.

Agree that size of the plant isn't relative to when to harden off. One can harden off at any size. But will a 2" plant tolerate it as well as a 6-8" plant? No.

But size is relative to when to transplant to the garden or container. So is weather. They determine the date they should be transplanted out. Hardening off begins approx. 1 week prior to that transplant date if the weather cooperates. If not then both hardening off and planting out may be delayed.

Dave