How much ice?

[PRE66_6TART]

I have been fermenting my ales in 5 gallon glass carboys in my laundry room, where it stays a constant 68 degrees. According to my stick-on thermometer my temps haven't been getting above 70 so far, but I've been under-pitching, which I plan to rectify. I'd like to start using an ice bath to get the temps down a bit to the low 60's for the first few days, using the frozen water bottle technique. I have a couple of those big plastic storage bins I can use. Anyone have experience with this who can tell me how much ice I would need to start with?

 

[WoodlandBrew]

That's exactly how I did it. Search for "swamp cooler" on my blog. It tells you how much ice it takes to drop the temperature.