Purely as a guess, but I would imagine it would be in seconds. The reason being, is that large differences are simply signified as 'T' - this assumingly stands for 'Total' i.e. you cannot practically draw a line down from one devices curve down to the other.
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local