It doesn't matter how long you practice: it matters that you do.

Once, I had dinner with some colleagues from another agency, including a senior person who was visiting from Singapore. The conversation turned around our hobbies. I happened to mention that I like to meditate. 

Senior Person: "Oh. How long do you meditate each day?"

Me: "Er. It depends. Usually around 15 mins?"

Senior Person: -pause- "That's not very long...." 

Haha. She's right. Undoubtedly, it seems like 15 mins isn't a very long time, especially every day. Later I found out that she does an hour of yoga daily. Maybe that's why she was underwhelmed. :)

I didn't want to sound like a smart aleck, so I kept quiet, but I couldn't help but be amused by her response. 

Here's what Ajahn Chah has to say about the need to sit for long periods of time:

"Sitting for hours on end is not necessary. Some people think that the longer you can sit, the wiser you must be. I have seen chickens sit on their nests for days on end!" 

(The rest of this interesting interview with Ajahn Chah can be found here.)

What matters isn't how long you practice, but that you do that as often as you can. 15 mins a day is fine. 5 mins a day is also fine. Ideally, you should build up to the state where your mindfulness is there from the moment you wake until you sleep. But before you get there, just keep plugging at it whenever you can. 

:)