Lengthy, long...
The opposite could be temporary, interim, or brief.
Lenghty
short
Brief, concise.
In length, the opposite of long is short. In time, the opposite of long may be short or brief.
The opposite of a prolonged activity is a brief one. Spending the summer in a place could have the opposite "a brief visit." The opposite of engaging in the same activity all summer would be doing many and varied things.
brief in a hurry limited time a little while a brief time
In length, the opposite of long is short. In time, the opposite of long may be short or brief.
The opposite of enduring (lasting) is ephemeral, brief, or fleeting. The opposite of the verb enduring (persevering) is surrendering, giving up, or quitting.
Oxymoron
There is no opposite of the noun "longevity" meaning lifespan. However, the opposite of longevity meaning endurance could be instability or ephemerality. Also, the opposite of long-lived (lasting) could be short-lived, brief, or ephemeral.
The opposite of summary is elaboration. While a summary provides a brief overview of something, elaboration involves expanding on the details and providing more in-depth information.