usually, as I know, Visa is issued for 2 weeks upon arrival, right at the airport. But later you can extend it I think up to 1 month.
Sometimes they give you Visa for 3 weeks right away (I got one on my first visit to Qatar 1 year ago), and when I came here next time, i got Visa for 1 month right away. But usually, as I've said, to us/canada citizens they give Visa for 2 weeks, with the option to extend it up to 1 month.