I've just begun Keldorn's Quest about his wife's infidelity due to his never being a husband/father. I am to meet him in 3 hours in the Hall of tthe Radiant Heart i think its called. How long will he remain there? I don't want to miss him as he is pretty useful for my party composition. I don't see him anywhere though its possible that the time hasn't passed yet. Can I just go ahead and do another side quest and come back to Keldorn after the side quest?

There is no time limit. He will appear there after 3 game hours and stay there until you return (or not).