Tryndamere Ult Timing

Experience Watch how fast your health drains, watch out for execution skills like riven's R, garen's R, etc Basically keep track of the enemy champion's skill and their AA dmg

P.s. Better to use the ult too early than too late

As was said it comes from experience and knowing abilities. It is a major factor that differentiates good tryndamere players. Also need to remember that there is a .5 second activation time. ( people who say high ping doesn't matter can suck it)