For most plants, annuals or perennials, give them about 4 to 6 weeks of growth under lights before planting them out after the last frost.

So for example if the last frost date for your area is April 1 and your seeds take 2 weeks to germinate, then you could sow them on February 1; expect them to germinate around Valentine's Day; begin hardening the seedlings off by putting them outdoors on non-freezing days starting in mid-March; and plant them out during the first week of April.

It depends if it is a cool or warm weather crop. cabbage onion and lettuce can take a small frost. I will be starting my onions in 2-3 weeks. but my tomatoes at the start of march will be ok.