Sideburns should end where they are fullest. Anywhere between the mid-ear or a little bit higher. The key is to do the "Frankenstein Test" to make certain the side burns are level with one other. Looking at a mirror using two pointing fingers from each hand and matching them up to make sure they are the same level.

Where do sideburns end and beard start?

That perfect length can be found by using the following method: Take a look at the inside of your ear- it has a little circular notch at the bottom. The bottom of that notch is where your sideburn should end. That is where you bring the edge of your razor to when you are shaving.

Where do sideburns start long hair?

So long as it looks natural, the length is totally up to you. In terms of the vertical length, you want the sideburns to start in the middle of your ear or lower. If they end too high, your sideburns will stand out.