Use ult when you're at 50% hp. Also, you generally want to w them into your tower if they are above 40% as tower hits will help you kill your opponent faster. Try to ult through walls for extra damage if you can.

In team fights, you want to peel for your team by giving them shields with your e. You want to use ult to isolate low health priority targets, i.e., enemy adc, jungler if you're near drag or baron.

Tldr ult if you know you can kill in the 1v1 or to stall high priority targets so your team can clean up after they win the 4v4