The real problem with leaving beer in primary for too long isn't the yeast, it's the hops. If you're dry-hopping, leaving the hops in the fermenter for too long can cause vegetal or grassy off-flavors.
The only way to be sure that fermentation has completed is by measuring the specific gravity. Ten days after pitching the yeast, you should take a sample of beer from the fermenter and measure the gravity. You then take another reading two days later, if both readings are the same fermentation has stopped.