The best time to see the northern lights is in August. The Auroras are at their peak during this month, but the nights are not as dark. You will have more chances of seeing them in August compared to other months or days of the year. The Aurora borealis can be seen from mid-July until early March depending on weather conditions and sky transparency for your location.
How long does it take to see the northern lights? It depends on where you are located around Earth, how far north or south you are, how clear your skies are etc.. but you should expect a few minutes at least! Depends also on if there is any wind which may make viewing difficult!