Why does the git key disappear from ssh-agent?

Question

git

On the VPS, I generated an SSH key, added it to Github as expected, checked the connection, everything works. I go the next day - there is no connection, because ssh-agent does not see the key. I had to re-add (as many as 3 commands to print). At first I thought that maybe the VPS was rebooting, but no - my application is still running (it would have crashed).
Today it happened for the third time. What it is? Why do I need to add a key to ssh-agent every time and how to fix it?

Answer 1

man ssh-agent
man ssh-add
see switch -t life