I sorta rotate feedings around the house and I have about 6? in the house I play in. Only cowplants that seem to be planted on a certain corner of my house tend to die out while the other few are troopers and seem to stay alive since I've planted them for a while. (2 of them are fairly new, though.)
They can die by overfeeding if I remember right. There was a thread where simmers mentioned cowplant behavior somewhere...
They usually eat twice a day, and someone mentioned they would stick out a cake whenever you have visitors, too. So it'd be best if they had some sort of reliant schedule. They also don't usually eat the sims that feed them. (Or digest?)
They also won't digest sims that have a green bladder, so if you're worried about your sims dying from them, you can always make sure their hunger doesn't drop too low.
I'm not sure what would increase the duration of them being alive, but I can play my house sometime to see how long it takes a new cowplant to live since mine seem pretty hardy these days lol