Credit: Instagram

What is the right length of a sharable, mobile video clip?

Ask the creators of Twitter's Vine app, and they will tell you six seconds. Ask Kevin Systrom, the founder of Instagram, and he'll say 15 seconds. Today Instagram added video features to its photo-sharing app, which allows users to record anywhere from three and 15 seconds of video.

"In some ways, it's arbitrary," Systrom told "Nightline" anchor Bill Weir during an exclusive interview. "But in some ways it's that just right moment between too short that feels constraining and too long, which takes too long to download over a mobile network. Fifteen seconds is this nice balance of capturing a moment, and also letting you download it quickly."

All the Details on Instagram's New Video Features

While most will say 15 seconds is still too short or limiting, heavy users of Vine might see it as too long. "Is 15 seconds too much?" technology website the Verge asks. For many, the six-second Vine length is similar to animated photos - it lets you capture a specific instance or moment and then the service loops it. "Like Tweets, the brevity of videos on Vine (six seconds or less) inspires creativity," Twitter VP of Product Michael Sippey said when the service was first introduced.

Systrom says Instagram's short video length was designed to inspire the same simplicity. "Instagram's all about being simple. And what we wanted to do is focus on 15 seconds of simple video. So I think we'll start there, and we'll see what the community decides," Systrom said when asked if he would consider upping the limit to half a minute.

Other experts are suggesting that 15 seconds might be the perfect length for advertising. As Quartz points out, TV are frequently 15 seconds in length.

There could also be some other significance to 15 for the company, which was acquired by Facebook in April 2012. "When we joined [Facebook], we were 15 people, 15 people working on a product supporting millions of people," Systrom said. "Now we're 45."