The first year this worked wonderfully and in the spring we harvested quite a bit of lettuce before anything was ready from the regular garden. I think I was just lucky that year because I had no idea of when to start the seedlings and no problem with slugs. In the intervening years I didn't plant soon enough (last year) or slugs had a feast. It must be a real treat for them to have a protected, relatively warm place with delicious young seedlings to feast on.
This year I either planted early enough or the unusually warm November gave the seedlings enough time to mature. But, as a number of people have experienced, slugs and snails have had a very productive year. I originally set out 35 seedlings and a little later replaced six or seven of them to maintain the 35 seedling number. But as some of those began to disappear I discovered that slugs were the problem. I had left the black six-packs with a few remaining seedlings in the frame and discovered that slugs were spending their daylight hours under the six packs and in the grooves between the cells of the six pack. Obviously slug control was needed. I set out a piece of board with black plastic stapled to it in the middle of the seedlings and a small black plastic tray in a corner of the bed. Each day I go out, remove the glass cover and transfer whatever slugs I find into salt water. I am surprized at the number of slugs that I have removed from this relatively small area - somewhere between 40 and 50. I don't know whether they are immigating into the frame from outside, or there are eggs hatching in the soil, or there are just that many slugs there. When I go two days without finding slugs I think the battle is over but then go out the next day and find three or four more.
I have also scattered crushed egg shells around the seedlings and for the past week or ten days the number of healthy looking plants has held at 24. I don't know why I didn't do that when I planted the seedlings because I did that earlier with Chinese cabbages. What I did do initially was spread some wheat bran around the seedlings. Rumors that slugs eat the bran and die didn't work for me. The bran absorbed moisture and got crusty and needed to be removed.
As to the correct planting time for the seedlings, I just read in Eliot Coleman's "The Winter Harvest Handbook" that plants you want to harvest through the winter need to have almost reached maturity before the day length becomes shorter than ten hours. In Syracuse that would be November 8th. I started this year's lettuce on September 23rd and transplanted them into the frame around October 28th. That seems about right, although maybe starting the seeds a week earlier would be better. I recall also reading recently that September 15th is suggested.