Random selecting 20% between a time frame

I have a lot of recordings to analyze but not enough time to go through the whole of each recording. So now what I would like to do in R is to put a csv document in there. With the filename and the duration of the recording. As example:

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Now what I would like R to do is randomly select 20 % of the duration for each file. And then put these numbers in a new table with the file name. Is this possible to do in R?

Cheers, Simone