50 feet for RS-232. This might be an old spec that was created before Cat5 was being used for it though. You might be able to double that using Cat5. With IR i have seen passive baluns with IR connections that say 1000' max distance so even if you cut that in half your looking at 500'. I have a few projects with IR being sent over Cat5 as far as 200'.

Whats the application? Crestron has room solution boxes that have IR and RS-232 connections on them that let you use Cresnet to wire back to the head end. Just Add Power HDMI over IP has an IR port also. Products like this allow you to circumvent the spec.