Once you open a bag of Doritos the opened bag of Doritos will last for up to 2 weeks before they go stale and lose their flavor.
You can still eat the Doritos after 2 weeks but they won't taste as good and may be a bit tough to chew but they are not unsafe to eat.
If you open a bag of Doritos you should use a bag clip to seal the bag to help keep the Doritos fresher longer but if you want the best freshness from the Doritos you should eat them before 2 weeks.
I've eaten stale chips before including Doritos because I didn't want to waste them but they were not as good or as enjoyable.