erc-server-flood-margin

erc-server-flood-margin is a variable defined in `erc-backend.el'.
Its value is

10

Documentation:
A margin on how much excess data we send.
The flood protection algorithm of ERC works like the one
detailed in RFC 2813, section 5.8 "Flood control of clients".

* If `erc-server-flood-last-message' is less than the current
time, set it equal.
* While `erc-server-flood-last-message' is less than
`erc-server-flood-margin' seconds ahead of the current
time, send a message, and increase
`erc-server-flood-last-message' by
`erc-server-flood-penalty' for each message.

You can customize this variable.