Trace amounts. You will not see any difference on your data bill due to OtO!
On a daily basis, OtO uses very small amounts of data to check the weather and keep it's schedule up to date.
During peak season, we may issue over the air updates to OtO devices usually once a month. Each of these updates is usually at the very most 1 MB.
That being said, OtO does require a consistent and strong connection to the internet. Read more about Wi-Fi here!