I don't have my seed packets handy right now so I won't be able to be specific, but I've been starting impatiens from seed for several years now and haven't noticed that they take any longer than it says on the pkt. of seeds to germinate. I think it's around 10 days, but mine normally germinate more quickly because I put the container about 4' away from our wood stove.

Our last frost date is around May 25, but yours may be a little earlier even though you are also in zone 5. I would suggest you count back 10 to 12 weeks from your last frost date and sow the impatiens seeds then. For me that would be mid March. That should give you plenty of time to grow them out to the size you want before planting out. If you want bushy plants it's important to keep the light source about 4" above the plants at all times. I use grow lights and move the lights up as the plants grow. If you are going to grow them in a greenhouse I guess they would get enough light that way too. I don't recall pinching back my impatiens as they grow slowly.

I use soiless potting medium to start my seedlings and fill small containers, about 5" x 9" x 3" to the top with the seed starting mix. You can use other type containers but be sure they have good drainage holes in the bottom. Be sure to moisten your soiless mixture first as stated on the bag so it is just slightly damp. Then after sprinkling the tiny seeds on the damp mixture, I press in lightly and place the container in a pan of water until it feels wet to the touch on top. Then drain it in the sink for about 15 min. Then I label the container and place it in a plastic bag. Fold the excess bag under the container, cut a few slits in the top of the bag and put the container in a warm place to germinate. I would say about 75 degrees would be good. If it's not warm enough, it will take longer to germinate. Once you see the tiny sprouts coming up, remove the plastic bag and put the container under lights, if that's what you're going to use. I use a diluted fertilizer, 1/4 tsp. to a gallon of water, every time I water, and I ALWAYS water from the bottom by putting the container in a pan of water. An old 9 x 13 baking pan works good for this.

After the seedlings have two or three true leaves, that's the ones that come after the first two, then I pick out the seedlings into individual plastic cells using Pro Mix potting soil. I continue then to grow them under the lights, watering them from the bottom, and fertilizing as stated before, until it's time to plant them outside. If you're not familiar with how to do this, you can find the info in a gardening book at your local library. I've seen it in several that have to do with propagating.

When your ready to plant them outside, be sure to harden them off first to acclimate them to the outdoors. That's another subject and you may already know abut it so I'm won't elaborate.

Hope this helps.