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 that bounces between allied and enemy champions. This ability can only bounce to each target once and hits up to 3 targets. The damage and heal are modified by -15% (+ 7.5% per 100 AP) with each bounce.