Replace the last colon with the decimal separator of you local. Then you can use TIMEVALUE to get the internal number of the time, which is a decimal number in unit “day”. Then multiply by
24*60*60 (edited for avoiding misunderstandings; lupp) to get the seconds.
Example: I assume, that the text 7:20:45:53 is in cell A1.
Herein is “,” the decimal separator for me in Germany.
It assumes, that the hours exists, so that the position of the third colon is greater than 7.