The short answer: you can't.
However:
You should also include a flood="x" line which indicates the number of
lines a user may place into their buffer at once before they are
disconnected for excess flood. This feature can not be disabled,
however it can be set to extremely high values, rendering it
effectively disabled. A recommended value is 10. A counter is
maintained for each user which is reset every 'threshold' seconds and
specifying this threshold value with threshold="X" indicates how often
the counter is reset. For example, with flood="5" and threshold="8",
the user may not send more than 5 lines in 8 secs.
(Source: https://brainbox.cc/stash/projects/INSP/repos/inspircd/browse/docs/inspircd.conf.example?at=a3491c9be4bf2c9899bde9c85da9e7d2c4fd694e)
This paragraph refers to the "connect" tag in /etc/inspircd/inspircd.conf which might look like this:
<connect allow="*" timeout="60"
flood="999999" threshold="1"
pingfreq="120"
sendq="262144" recvq="8192"
localmax="3" globalmax="3">
Unfortunately, I have not yet tried this out myself. Apparently, InspIRCd has every conceivable feature except good documentation, and having a "flood" Attribute in a "connect" tags made me assume that it refers to Connection flooding, not message flooding. So after three days of searching, I just built rate limiting into my bot.