2. For first time, I'd do like 10-15 minutes max, then warm the water up to between 50 and 70 and finish up with another 10-20 minutes. Make sure someone knows what you are doing. You don't want to get hypothermia. But if you've done 40-50 degrees already you should be fine.
3. Not sure what you are asking. If your tap water is in the 60's, you may need a lot of ice depending on tub size. 50lbs or so.
Let us know what you think!