How long before you get sunburnt?

If someone is pretty white how long before they start to burn? My upper arms were pretty white and i was doing some gardening with a singlet (wife beater?) on and was out in the sun for about 2 hours. Got burnt pretty bad. Would you still get burnt for an hour, or should you be ok? 30 mins? umnig you werent/couldnt have suncream on or be covered up.