erc-lurker-threshold-time

erc-lurker-threshold-time is a variable defined in `erc.el'.
Its value is

86400

Documentation:
Nicks from which no PRIVMSGs have been received within this
interval (in units of seconds) are considered lurkers by
`erc-lurker-p' and as a result their messages of types in
`erc-lurker-hide-list' will be hidden.

You can customize this variable.