Download Limit Concern

Just got the RB 951G Mikrotik Router and configured for hotspot as in the e-book downloaded. I wanted to limit the download to say 20 mb so i indicated the product of 1024x1024x20 in the “transfer” but nothing in “upload” and “download” but whenever I sell the hotspot ticket, it logs customers with laptop or even the PC out automatically or midway into their surfing and has to give them another ticket. This is giving me sleepless night as the purpose of limiting users is being defeated. Please what do I do to address this issue.