What's up crisc! I've used flora a couple of times. Good stuff, I just am wondering which type your using, 1 part or the 3 part. As for your question, I'd start after about 2 weeks, maybe longer, depending on how fast they're growing. If its a dim light then they might grow slow. Try switching to flo's. I've tried it all and the best for seedlings are the flo's. Get some T5's and keep em on all day. That'll get you there quicker and then start out about 200ppm on the feeding schedule assuming your growing in a soilless medium.