You could begin to hardened off any time, weather and shelter permitting. It isn't size that determines when to harden-off but the conditions you do it in. Any size plant can get accustomed to the outdoors if you keep it proteted properly.

And doing so will help speed up its growth. But at only 3.5" tall I prefer to have it grow to at least 6" tall before transplanting it to the garden.

Dave

Dave, thank you. So I understand you correctly, are you saying that I can/should start to slowly harden off now. But rather than the usual (maybe 2 weeks) it normally would take to harden off, I might be best waiting even a 3rd week if the plant is less than 6" after 2 weeks?

You are talking about 2 different things. Hardening off and ideal transplanting size.

Just as plant size does not determine when to harden off, neither does length of time. It is the method used to harden off, the environment and protection provided, that controls both. So any size plant can be hardened off in 7 days IF you create a good hardening off environment and don't rush the process.

Ideal transplant size varies from vegetable to vegetable but pepper plants have difficulty adjusting to the process if they are overly small so 6"+ is a good size for doing it because by then they have usually developed a good root system that smaller plants simply don't have.

So if in 2 weeks your pepper plant is still not 6" tall then yes, you will need to wait longer or discover what is keeping it from growing.

Dave