It's been a couple of years since I've used my cold frame, but I remember putting my seedlings into mine as early as end March/early April. I would say now would be a good time to put them in your cold frame.

I don't have a 'scientific' method, exactly, but I just put my trays in, and kept the lid ajar if it was a warmish day out; then closed the lid at night. I did this until the seedlings were hardened off, and there was no longer a risk of freezing temps; then I took them out of the cold frame, and put them in a sheltered spot (outside) until I could plant them out. Just remember to keep them watered.

When to place seedlings in the cold frame depends on both weather and the type of annuals involved.

Varieties that can handle a bit of frost (i.e. "half-hardy" annuals such as petunias) can go out into the cold frame while there's still a chance of a few light frosts (they need to be hardened off with the lid closed during frosty weather, before you can risk exposing them to frost in the open). Frost-sensitive annuals should go out later; even if the cold frame protects them from frost, sustained very cool temps could set them back.

Eric, I've never had a problem putting out any seedlings as early as now (mid-April), or even earlier. I find just keeping the lid closed, or even pryed open slightly, is ample protection, if the weather isn't seasonal.

I find the seedlings do *much* better outside in the cold frame, than inside under the lights. Just my experience, of course. You can always experiment with a small amount of your seedlings if you are somewhat nervous about putting them out.

Months don't mean much in different parts of the country. Temperatures would be much more helpful. Like how low can the outdoor temp go and the seedlings will survive in a cold frame?