The distance limits depend on how do you hook up the RS232 devices to the CAT5E cable.
RS232 is a common mode transmission protocol, and if it is just a straight connection, it severely limits the distance, and you can only get 30-90 feet (it depends on the actual equipment's ability to drive the signal).
In case you are using passive baluns (balance-to-unbalance transformers), you can greatly increase the distance up to 2 miles. If you are using active (powered) baluns (line drivers rather), you can go as far as 10 miles and up. This would not be relevant to CAT5E, of course, as CAT5E can only go for about 300 ft, but his is the idea - the common mode signal has to be converted into balanced in order to cover greater distances.
Dmitri Abaimov, RCDD