Cold frames have multiple uses. They are a great place to harden off seedlings started indoors, and what I do is put my plants inside, adjust the door for how hot or cold it gets and cover the glass top with white plastic for a few days, then over the next week cut more and more of the covering away to allow increasing amounts of sunlight to contact the seedlings. I also use my cold frame to grow plants in. Before it is warm enough to grow outside, I grow cold tolerant crops inside (lettuce and other greens mostly). You need to adjust the ventilation as it gets hot or cold inside, but there is no need to take plants in and out once they are in there for hardening off. Hope that helps.

Thank you. How cold do you let it get inside overnight b4 you move them back inside (i.e. for a cold snap)?

I don't generally put tender plants into the cold frame until the weather is threatening to be frosty but the worst of cold temps are past. So for me in zone 3, last frost date is about May 30th so I utilize my cold frames the whole month of May. I don't worry about frost as long as the cold frame is closed, and that has been enough to protect even tender plants. I have never moved plants back indoors once they are out in the cold frame. I just carefully open and close the top to prevent either cooking or freezing the plants. There are different types of cold frames, and your results may depend on how drafty or sturdy yours is.

Using cold frames is a hands on learning experience. I have several of similar design and they're all somewhat different as to how much heat they hold. Here in zone 5 I probably won't start putting tender plants out until April. I have no problem putting cold tolerant plants out in the frames practically anytime during the cold season.

Generally the seedlings go in the frame where there is full shade for a few days, then partial shade and finally full sun. Not unlike hardening off in the great outdoors.

If it gets really cold I cover the top with space blanket tarps and or old sleeping bags. I also do that with my tunnels. My roommate says it looks like a shantytown.

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Some folks put milk jugs full of water in their frames early in the season, to soak heat up in daytime and give off heat at night.