In this wikipedia article, it describes planetary nebula as "short-lived," but I'm not sure in relation to what? This could mean billions of years in some contexts.
How long is short-lived?
Bonus if there are any useful rules of thumb in astronomy terms for future uses (how stars timelines are described, versus galaxies, etc.)
In the case of planetary nebulae we are talking of ~10,000 years (follow the link to the main article on Planetary Nebulae).
In general a short lifetime for a phenomena means a time short compared to the total life of the entity it is related to. It is an informal term and does not have a fixed maximum value, but 10,000 years is a short time on most astronomical scales.