you can try things like 40s (40 seconds) and 2m (2 minutes), you can work with:
s: seconds,
m: minutes,
h: hours,
d: days and
w: weeks.
mute no longer accepts just a number, you need to specify one of the time modifiers mentioned above.
so:
will no longer work, but
will.
I'm not sure if bug or feature, though. I suppose it should be mentioned in /help mute either way.