Tricia - timely question as we just had ours installed over the last two hours. Ours runs the entire width of the upper cabinets. You could do less but I think the quality of light looks really nice with the entire space well lit.
Hope that helps!

Hi Marina -
May I asked what type you chose and how you like'em? Of all the kitchen decisions, lighting is giving me the biggest headache!

I just installed LED strips the full width of the uppers and they look great. If you haven't seen these neat rolls of lights that you can cut to length, try Home Depot. They're cheap to operate and give an amazing work light. As well they almost don't need a valence to hide them.

Tricia - I just checked - they are NSL (National Specialty Lighting). I think there are less expensive options. I have been involved in most aspects of our lighting decisions in the house, but in this one area I let the architect who has specified our lighting make the decision. So I can't be very helpful except to say the quality of the light is nice, and we specified switched UCLs. What moebus says is something to consider - the type we installed need to be hidden by trim under the upper cabinets.

You should run the lights the entire length of the cabinet. The purpose of undercabinet lighting is task lighting on your counters. You are not doing tasks only under a 12" spot. You will be using your entire counter, so light all of it.

Furthermore, you want to install the UCL's toward the front of your upper cabinets. It doesn't have to be exactly at the front of the cabinets, but as close as you can. Don't install the lights against the back wall under the cabinets. Here are the reasons: (1) your goal is to shed light on the counters; if you install the UCL's toward the back, you are mostly lighting your backsplash. That's a lot of money to spend just to light your backsplash, and not accomplish what the UCL's are meant for, which is task lighting on your counters. (2) Your cabinetry should come with a light rail, which is a strip matching your cabinets, installed along the front edge of your upper cabinets (can be elaborate or a simple strip). It's what MarinaGal referred to as "trim". The light rail's purpose is to hide your undercabinet lighting, which should be installed behind the light rail, not toward the back of your cabinetry. You are defeating the purpose of your light rail if you install the UCL's toward the back. (3) If you install the UCL's toward the back, they will be visible from a seated position in your kitchen, or nearby dining. If you install them toward the front, they will not be visible, because they will be behind the light rail.

Don't listen to electricians who want to install UCL toward the back, and who might say something like "the wires will show". The wires will NOT show, if the electrician has even basic skills in installing undercabinet lighting. Electricians want to install UCL in the back cause it's easier to do it, less craning of the neck for them, and it's easier to get their hand up there when it's installed toward the back. So, they have an easier time, maybe save themselves 15 minutes of installation, while you have to live YEARS with the UCL. Just be firm about it.

Awesome suggestions Fixin'! I just this exact conversation with our electrician. I can't figure out why anyone would put the lights in the back, so I asked for them to be at the front. Before they are installed for real I intend to hold the light bars in a few different spots, with them turned on, to see how the lights falls on the countertop. But certainly they'll be at or near the front.

Excellent point by Alwaysfixin - I did have to have my electrician move the first UCL fixtures he installed forward. He had started to install them toward the back just as Alwaysfixin warned.

Moebus- Could you tell me which led light you are refering to at Home Depot? Are they direct wire? I would like leds where I don't have to have a light rail.
TIA Parts

Parts-I don't have the details, I just was astonished to see flat tape LEDs on an end aisle display in Home Depot. It is a Canadian store and some of their products vary from the US.
That being said, I used LED tape from Seagull. www.seagulllighting.com and found it was excellent for my purposes as I had two long runs of 6 and 10 feet. It is tape so there is no rail and it is direct wired so switchable and also dimmable if you wish. It should work with the adhesive backing but I added the tiny clips provided as insurance. As an aside, I suggest using the warmest colour that you can.
Trust that helps.

Thanks everyone. I actually saw some of the tape lighting at Home Depot the other day. They told me they just came in so all this stuff is new to them as well.