If you take promethazine with it, do NOT take it until the codeine is fully acting, promethazine is a CYP-P450-2D6 inhibitor (or substrate, forgot which, maybe both) and it will DEFINATELY block conversion of codeine to morphine, you get more sedation but far less euphoria thus.
Taking it after the codeine has kicked in, you will gain the beneficial effect of both drugs.
Wouldn't drink the full bottle, 500mg is the absolute maximum of codeine you can convert, so a 600ml bottle, you WILL have 100mg left for future use if you choose to save that last bit, and if you drink the full bottle, you WILL waste that 100mg.
All it will do, after that 500mg is increase histamine release and make you itch more.