I have the RB751U Mikrotik running as a hotspot for a 55 room hotel.

The clients have complained that if they walk away from their session for a while and come back, they have
to login again.

In Winbox, IP, HOtspot, User Profile, the Session Timeout is 0

Why are their sessions timing out if the timeout setting is Zero ?

In hotspot profile enable also mac-cookie and cookie so they dont have to input their login info when they go away and come back. Also in user profile check idle and session timeout