How long it takes to sunburn by UV Index Ratings

First let's start with a means of finding your UV index. Check your UV index here.

0-2 Minimal UV Exposure, over 60 minutes to sunburn.
A wide brimmed hat should do, and sunglasses if applicable. 
3-4 Low, sunburn can occur in approximately 45-60 minutes or more.
Wear a hat and protect your skin using clothing UPF 15 and SPF 15 on any exposed skin every 2 hours.
5-6 Moderate, sunburn can occur within 30 minutes.
Wear a minimum of UPF 15 clothing, UPF 20-24 is recommended, and polarized sunglasses to protect your eyes. Sunblock SPF 15-30 should be applied to any exposed skin, every 2 hours.
7-9 High, sunburn can occur within 15-20 minutes.
 Advised to stay inside between peak times of 10:00AM - 4:00PM. Wear UPF 40-50+ clothing, hat, polarized sunglasses and SPF 45 to any exposed skin every 2 hours.
10+ Very high, sunburn can occur in less than 15 minutes
 It is recommended you stay indoors. If you go outside, try to stay in a shaded area, keep yourself hydrated, wear SPF 40-50+ clothing, wide brimmed hat, polarized sunglasses and SPF 45 on any exposed skin every 2 hours.