This is the journeyplanner with all schedules:
I've just guessed a central point in Voorburg and the beach in Scheveningen which is 35 minute travel time.
There are various ways to get there, depending on where you leave exactly and where you want to arrive exactly. So you'll have to try that out yourself by using the journeyplanner.
In general though, most itineraries are 35 minutes including a little bit of walking here and there.
The cost depends on which 'strippenkaart' you have bought.
At the most the trip would be 3.20 euro one-way (which is 4 strips on your strippenkaart).
For information about rates and strippenkaart:
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