It depends on how warm/cold the greenhouse gets and it depends on what type of seedlings they are. It also would depend on what the weather is like when you move the plants out. Every type of plant has minimum temperatures they can tolerate, so you have to know (a) what is the lowest temperature that specific type of plant can handle, and (b) how cold is your greenhouse getting at night and how hot is it getting during the day. A Min-Max thermometer in the greenhouse, monitored daily, will help you figure out (b). The Min-Max thermometer also will help you figure out how many degrees higher than the outside air temperature the heater is able to achieve inside the greenhouse when you have it set on various settings. Just because you set it at a certain temperature, that does not mean it will keep the greenhouse at that temperature. It should, but much would depend on how much colder the outside air is and on how well your greenhouse holds (or doesn't hold) heat. You don't have to keep the greenhouse hot, but if you're keeping warm-season plant seedlings like tomatoes in the greenhouse, if you let the temperature go below 50 degrees, their growth will slow down a great deal. If you let the temperature go below 32 degrees, the tomato plants can be damaged or killed. Some tomato plants can briefly withstand temperatures as low as 30 degrees, but not for an infinite period of time.

Remember that seedlings germinated indoors have not been exposed to cold temperatures during the brief time they were sown/germinating/growing indoors and they certainly can die when moved out into significantly colder temperatures. You have to gradually expose them to colder temperatures to build their tolerance of it in the same way that you have to harden them to increased light exposure. That sort of hardening off to colder temperatures is more important with warm-season seedlings than with cold-season seedlings which can tolerate colder air naturally.

I usually keep my seedlings (hot or cold season) indoors under lights until at least 3 weeks after they germinated, then I harden them off and then I move them to the greenhouse to stay until they go in the ground. I don't heat the greenhouse though, so I won't move them out if I am not sure the greenhouse will stay warm enough for them at night. There's ways to work around indoor greenhouse temperatures going too low, like using row covers for an extra layer of protection in the cold weather, or having a cold frame inside a greenhouse for the same reason. If you cannot do something to ensure the seedlings don't get too cold, it'd be better to wintersow them (wintersown.org) and expose them to the cold from day 1 so that they aren't used to growing in abnormally warm indoor temperatures.

The amount that a greenhouse heats up during the day in full sun if it is closed up gives me more trouble than how low it drops at night. That's a problem I create for myself, though, if I forget to open the doors and the vents early in the day on sunny winter days. The corollary to that is that I also have to close the doors and vents early enough in the afternoon that they hold enough heat inside the greenhouse all night. If I leave the doors and vents open in the unheated greenhouse until sunset on a winter afternoon, there's not going to be much heat left inside the house by the time I close them. On an average day, I close them between 3 and 4 pm, depending on how cold the day has been and how cold the night is going to be. If a very cold night is forecast, I might close the doors and vents at 1 or 2 pm to hold in whatever heat there is available.

I keep water-filled containers in my greenhouse to serve as solar heat collectors, and they keep the greenhouse from dropping too low at night on all but the most bitterly cold nights. I've overwintered citrus trees in my greenhouse ever since we built it, and never have had a heater in there....but there have been times they've gotten colder than I liked. I've never had tomato plants or pepper plants freeze to death in the greenhouse, for what it is worth, but I'm also very vigilant about not moving them out of the house too early.

With your greenhouse, you'll learn as you go just like the rest of us have learned with ours. With each greenhouse, the temperature fluctuations experienced by each person in their specific location with their specific type of greenhouse and their specific weather is going to be highly variable, so there's no hard-and-fast guide or formula to follow. The Min-Max thermometer is the key to success. Without it, you're just guessing how hot or cold the greenhouse gets.

When you get a MIn-Max thermometer make sure you get one that goes below 32 F. The first one I got didn't. Then I found one at Lowe's that goes to zero F. That works much better for me in the greenhouse. I bought several and put one on my porch and 3 in the greenhouse at different places.

thank you both for the help.going to get the thermometer asap.