It's a bit wierd but, let's say you enter 10:55 in cell A1.
In cell B1, enter the formula for conversion: =A1*24
Excel's brainless 'Intellisense' will try to display it in HH:MM format which makes no sense: You need to change the number format to Number, 2 decimal places, and you will get 10.92
This will work with huge amounts of hours beyond 24.
Good luck...