erc-mode-line-away-status-format

erc-mode-line-away-status-format is a variable defined in `erc.el'.
Its value is

"(AWAY since %a %b %d %H:%M) "

Documentation:
When you're away on a server, this is shown in the mode line.
This should be a string with substitution variables recognized by
`format-time-string'.

You can customize this variable.