The ones I've used in the past only last maybe 15 or so seconds. The trick is to put your heating on recirculate, and air conditioning, then stick them full blast, then kick off the can. Shut all your doors and windows first then let your fans run full blast for about 5 - 10 minutes.
It's always best to change the interior air filter once a year as well. Most people don't even realise they have one. It's sometimes called a pollen filter.
I do both my vehicles once a year. I use Maguires or Halfords freshener cans.