There is no single answer to "how long does it take to get sunburned?" — the time depends almost entirely on two variables: the UV index where you're standing right now, and your Fitzpatrick skin type. At UV 3 with very fair skin, redness can begin in about 20 minutes. At UV 11 with the same skin, it can begin in under 10. At UV 3 with deeply pigmented skin, you may not see visible redness for over an hour.