My DD reads like crazy and has to keep track of time for our library summer reading program and for independent reading programs at school. I bought a few of these bookmark timers, as she will often have three books going at once. It's easy to keep track of time because all she has to do is push a button when she starts and stops reading. They are pretty inexpensive (about $9 each). You can find them at bookstores and office supply stores. Here's a link to Barnes and Noble for example:
http://www.barnesandnoble.com/p/home-gift-mark-my-time-assorted-bright-color-bookmark/19908485
