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.

You need `=TIMEVALUE(REPLACE(A1;FIND(":";A1;7) ;1;","))*24*60*60`

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.