Some factors that affect the time to freeze are;
- Volume of water in the bottle
- Size and shape of the bottle
- Temperature of water before it goes in the freezer
- Temperature of the freezer
- Position of the bottle in the freezer
- Impurities in the water
To determine the formula for your specific bottles;
- Make a note of the time
- Check your bottles after 90 minutes, and thereafter every 15 minutes until one freezes
- Mark the time of the frozen bottle, and subtract 15 minutes.
In my case, the first bottle froze after 2 hours and 45 minutes, and I found all the bottles worked consistently between 2-1/2 hours and 2 hours 45 minutes.