Hardening off can be a very important part of the seed starting process. I figure at least a week to do it.
Hardening off is another use for my XL dog crate. I put the crate out in the middle of the garden, put the pots in it, and drape a white sheet over the crate. Attached with clothes pins so it won't blow away.
The sheet breaks the wind and softens the direct sunlight on the plants. After a couple days, I take the sheet off for a couple hours. An overcast day is the best for their first foray into real sunlight. Each day, I leave the sheet off for a couple more hours. By the end of the week to ten days, They've been in direct sunlight all day, with the breeze.