Use a spreadsheet-style formula in your zap to generate a random number between 1 and anything you want - then delay your zap for that many minutes (or hours or days).
Using a "Formatter->Number->Spreadsheet-Style Formula" step, you canmanipulate numbers just like you would in Excel or Google Sheets.
In this case, we use the formula RAND()+RANDBETWEEN(X,Y) to generate a random number to many decimal places.
X = minimum delay
Y = maximum delay
Keep in mind that the minimum for a delay step in Zapier is 1 minute. The maximum is 1 month.
In the example below, we tell Zapier to generate a random number between 1 and 3.
This is what the result looks like (remember, it will be different each time the zap runs).
You then need to add a "Delay->Delay For" step to the zap, using the generated number as the value.