how much ice to use for an ice lock session?

Looking to do a little SB this week. I want to use an ice lock of the kind where you drop a chain or two in a bottle of water, then freeze the water bottle, being set loose when it melts. Last time i tried something like this, I used an old 35mm film container and the ice in it lasted around 15 minutes before melting. Could anyone help me decide how much ice to use? About how long will it take for 8 oz of water to melt enough to get free? or 20oz? I’d like a 1-2 hour session to start with, so i just want to size everything right. What do you folks use? Thanks, —cal