Impossible to say really.

Due to variables like Defence, Attack, HP, Items and Abilities, Heavy Slam's damage will vary for every single different Pokemon.

Heavy Slam's damage is calculated like this;

120 base power, if target's weight is less than or equal to ⅕ (20%) of user's weight.

100 base power, if target's weight is greater than ⅕ (20%) of user's weight and less than or equal to ¼ (25%) of user's weight.

80 base power, if target's weight is greater than ¼ (25%) of user's weight and less than or equal to ⅓ (33.3%) of user's weight.

60 base power, if target's weight is greater than ⅓ (33.3%) of user's weight and less than or equal to ½ of user's weight (50%).

40 base power, if target's weight is greater than ½ (50%) of user's weight

So as long as your over 230KG... Snorlax isn't going to be doing alot to you. :P

And also Heavy Slam is great on Snorlax anyway Return/Body Slam > Heavy Slam any day because of stab ;-;