|Seedlings out being hardened off.|
I start when the plants are about seven weeks from sowing, especially the tomatoes (the superhot peppers I grow take a lot longer to germinate, so their timetable is extended.) Ideally you want the plants to have several pairs of leaves after the first cotyledons, and be about two to three inches tall (depending on the type.)
I try to start out on a slightly cloudy day with not too much wind. If it's windy, I put the plants in a box, so they can get light but not be too knocked about too much (or knocked over if I have them in cups, as shown at left.)
I begin with well-watered plants set outside to get 20 minutes of sunlight at about 11 am or 1 pm. Keep an eye on the plants the first time, if they start to get droopy, bring them in sooner. The next day I increase the time by five to ten minutes, depending on the time of day and the intensity of the sunlight. I also begin letting them get wind on them by the third time out.
This is not an exact thing, you have to wing it somewhat, depending on the variables and how the plants look. Pay attention to browning of leaves and droopiness. If need be, scale back. Better to take a bit longer than give a plant sunburn.
By the end of a week of this, your plants should be able to spend the night outdoors, as long as the temps don't drop below about 40 degrees F. Keep increasing the time in the sunlight until you've hit between three and four hours, and from that point onward, you should be able to leave them out all day and night.
Before transplanting the seedlings into your garden, it's helpful to know how warm the soil is. Not everyone wants to invest in a soil thermometer, but a good rule of thumb is to not put plants in the garden if the evening temperatures are still below 55 degrees F. That should mean your soil temperature is at 60 degrees F or above.
You can plant seedlings before that soil temp, but it could stunt their growth, and will certainly slow it. Better to wait a bit to be sure they get a good start.