I was always told "not more than 50 feet (15meters)", but 25 meters works too! At least for me. In an automatic scoring system I did for a bowling centre, I used RS232 links between the lane controllers at the back & the video controllers in the TVs. The worst problem is a thunder storm, which can kill 1 or 2 line driver/receiver chips.
It's best if the devices at each end of the line are on the same 230V circuit, which is the case here.

Regards,

Richard