The timer has options for 5-, 10-, 15-, 20-, and 30-minute blocks. Just rotate it to your preferred option and the display will begin its countdown. The timer also has four different volumes (though silent is an option) so you'll never miss it going off.
Promising review: "For someone who does almost all of their shopping on Amazon, I write incredibly few reviews. I need to take a few minutes to write this one though. This has become one of my favorite purchases ever. I value simple, clean design so I chose the all white but the function is what amazes. It was meant to help me 'chunk' time into blocks to make me more productive. It works. It works well for bookending when studying and needing to force yourself into breaks to help the material absorb. However, what I wasn't expecting was the fun game I now play with myself of, "Can I get this task done in 5 minutes? In 15 minutes? Before this alarm goes off?" Things that were tasks I put off are suddenly now done and fun memories. I'm not saying it will work for everyone, I'm not even saying it will work for me for the long term, but for the month I've had it, I've been more productive and enjoyed it more than ever. Now the question is, what can you get done in 5 minutes? 15 minutes? I highly recommend you find out." —Laurie
Get it from Amazon for $19.99 (available in 30- and 60-minute configurations and three colors).