That’s not how it works.
Mei’s freeze does damage in ticks. After 30 ticks, the freeze is complete. See more info here: (can’t include links, but check mei’s gamepedia page)
So, it still takes the exact same amount of damage to freeze an enemy.
Assuming the enemy is hit by every single “round”, they will take around 68 damage. If we do the math, that means it will take at least 1.5 seconds of freeze spray to freeze an enemy.
So, you can think of it this way: You need to deal AT LEAST 67.5 damage to an enemy to freeze them.
67.5(freeze) + 150(hs) = 210.5 damage minimum
- extra freeze time at 2.25 dmg/hit
There are other things like armor, shields, crit box location, etc. that need to be taken into consideration.
But basically, the longer you take to freeze an enemy, the more likely it is that a single hs will kill them, as longer freeze time equates to more damage.