What is meant by "short lived" in the duration of a planetary nebula?

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.