Are you using the stock weather widget? If so, I think it once every hour. If you want, there are much better weather widgets available on the market. Two good ones that i've used or still use are Beautiful widgets ( fully customizable with clock and weather skins) and also fancy widgets. Also fully customizable but only in the paid app. You can set the update interval to be every 5 minutes if you want.
Personally, I really like Weather Bug (paid). For where I live, it has a local station, so I get accurate temps and forecasts.
I also use BW's home weather widget (with time/date) with Google weather. The temps are generally within a couple degrees of each other, so I would recommend either.