Recently there have been a number of products launched that tell you the time in sun indicate the amount of UV irradiation. The direct measurement of the UV index makes it easier to tell how much sun you can get. The UV index varies with time of year, latitude, altitude, and local conditions such as cloud cover and pollution levels.
They UV monitors are divided into two types bracelet monitor and the meter. I checked out Amazon to see the most popular types. You can click on the images to take a look on the site.

Sunsense Wrist Bands -In a clever twist you apply your sunscreen on the band as well as yourself. When the color changes you are supposed to apply more sunscreen or get out of the sun. Unlike the UV monitors the bands do not compensate for your skin types, tell you the UV index or indicate the level of protection that you need. However they attached to you wherever you go and would be ideal for kids.
The package contains 7 wristbands and at current cost of $6.98 (eligible for Free Super Saver Shipping). I am assuming that they are one time use. The reviews on Amazon are mixed. Some people seemed to have trouble with the instructions. Two said they did not work for them.

The Q3 UV HAWK 2 Sunlight Meter with Waterproof Design directly measures amount of UV. The unit will tell you what SPF to apply for the local UV intensity and your skin tone.
It has a timer with an alarm to tell you to reapply sunscreen or get out of the sun. Small and lightweight, 2.3″ x 1.2″ x 0.5″ and 2 oz with a wrist strap.
This monitor currently costs $25.99 (with Free Super Saver Shipping). Available as a 6 pack, but cheaper to buy 6 of the single units. One very glowing review.

Oregon Scientific EB612 Personal UV Monitor with Exposure Timer is another UV monitor that in addition to monitoring UV intensity, will determine, for your skin type, the SPF of the sunscreen you should be using, and a timer for how long you can stay in the sun. Does not appear to waterproof.
In addition to a countdown timer it has a digital clock, monitors the current temperature and has a wrist strap.
Current price is $ 17.95 (eligible for Free Super Saver Shipping). Reviews pretty positive.
Now you have no excuse for getting burned.