To convert a time into a number of minutes, multiply by 1440, then format as
a simple number (comma style, for example) rather than a time. To do this
for a whole series of numbers, type the number 1440 in some random cell.
Copy it. Then select all the times you want converted and Edit > Paste
Special. Select the 'multiply' option and click OK. Then apply the
formatting.
BTW, the logic for 1440: Excel stores dates/times as numbers, specially
formatted, with the integer portion being days and the fraction portion being
parts of a day. There are 24 hours in a day, so multiply by 24 to get the
number of hours. Then multiply by 60 to convert hours to minutes.
--Bruce