how soon could I plant them outside? I'm thinking in terms of size: then they are 6 inches tall? When they have 2 sets of leaves?
Tomatoes benefit from deep transplanting. Bury them up to the first leaves and then they sprout new roots all along the stem, giving them the advantage of a greater ability to draw up water and nutrients.
If you started them now, you should be able to put them out in 5 to 6 weeks, if you are aiming for soonest. I'd plant them now, transplant into bigger p then I'd transplant them one time before transplanting out.
Here are tomato seedlings 4 weeks, 4 days old (seeded 23 January).
Here's them a week and a half ago, when I transplanted densely-seeded sprouts into pots at three weeks old from seed.
Here is a link that might be useful: Tomato seedlings at three weeks
Thanks. I'm specifically interested in growing Sungold (since they get such raves here), so if the nursery doesn't have seedlings tomorrow, I'll buy a pack of seeds and try starting them myself. Thanks for the encouragement, and wish me luck!
I find the key is keeping the heater pad thing under them the whole time. Consistent temperature is important. Also, plant 2 seeds in the bottom inch of a ~3-4 inch pot, keep the first sprout, pull out the second (ouch), fill dirt as the plant grows. Not disturbing the root "at all" plus the heat will get you real nice plants in 4-6 weeks.
Are you in zone 10? If so, it may be too late to start seeds even for cherry tomatoes like Sun Gold. You can try and see what happens.
The only place I've seen Sungold in plant form vs seeds is at my whole foods grocery store here in Austin. If you have one near you maybe you can grab a plant tomatotomata. They were 1 feet tall. I put mine nice and deep into the ground.
You can buy Sun Gold plants at Territorial Seed, but they don't start shipping until the first half of April. You'd be better off growing from seed.
I live where its a zone 11 like climate and now would be way too late to start seeds and I would imagine zone 10 would be too late as well... but not sure...