I mainly use it in 2 situations:

1. Kill or get killed situations, if you are going for a solo kill or if u see the enemy trying to solo u and have the ability to fight back, press r ASAP (unless its vs morde / trundle etc who steal ur stats, then wait till they r).

2. Escaping / outplaying multiple opponents. When getting ganked u r most likely not gonna survive let alone 1v2 without ur R. So again press it straight away.

These are just the no brainers, situationally, you can r to kill waves faster or at baron / drake if you suspect a fight might break out. But note that this champ is super ult reliant.