It seems to take ~0.875 seconds from cast to land (due to the cast time of the spell) regardless of distance. The AoE has a radius of ~162, making it impossible to escape only if an enemy is at 185 movement speed or less (325 movement speed slowed by 43% would put an enemy within this). This means that CC may be necessary to ensure that this CC lands. Rank 5 Tidecaller's Blessing with 160 AP could reach this slow value (though few enemies would likely have 325 movement speed by that time).
Ebb and Flow
COST: 70 / 85 / 100 / 115 / 130 mana
Active:Nami unleashes a stream of water onto the target champion or herself which then bounces to nearby unaffected champions up to twice, alternating between allies, including herself, and enemies.
Ebb and Flowheals allies and deals magic damage to enemies, with each bounce being 85% (+ 7.5% per 100 AP) as effective as the previous one.