/rubs hands together
The life span of a Spewli is entirely base on you.
Typically, a Spewli life span can range from 2 years to forever(while having their appearance still being alter base off that age).
Plus, you have to remember that conditions in nature are never perfect for very long. So, even though a Spewli has the ability to possibly live forever, something will eventually happen to kill it, like drought, fire, disease, or other Spewli using up all the soil nutrients.
So if you were to design like a Spewli with a elderly like apperance, then you can say that that Spewli lived for long period of time(wilthering flowers, dried up soiled skin, etc). It can also depends on what type of flower you base your Spewli to be.
Say if you base your Spewli's flower to a Kadupul flower.
Kadupul flowers are hella beautiful but also so rare for they bloom only at mid night and perish before the dawn.