Every bottle is going to be a little bit different.
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
The easiest way to find out the ideal freeze time for your bottles, is to start with bottles at room temperature, and put multiple bottles in the freezer at the same time. This will increase your chances for success.
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.
Your specific bottles should work within this window of opportunity.
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.