Is 28mm of rain a lot?
It's quite a lot, a little more than an inch. It would depend over what period of time it fell. 28mm in an hour is quite different to 28mm in three days.
This would really depend on where you live. For example, London vs the Sahara. It would be two totally different answers for the two locations
First of all, where in the world do you live and what time of year is it.