Thanks for watching, to answer your question, Swain's ultimate has infinite duration for as long as he damaging an enemy with it and also heals for every enemy taking damage within the radius, he has very overwhelming sustain and can turn team fights around if he's able to survive the initial burst of team fights as you can see :v

rylai's, riftmaker, crystal reflector, situational But for runes I used fleet footwork & the resolve tree for more survivability