PUZZLE 059 Sleeping Sand

This girl has two sand timers.
When she sees the sand in both of them finish falling to the bottom at the same time, it makes her sleep like a log.
One of the timers takes three minutes and one of them takes five minutes.
How many minutes would it take for the sand in both of them to finish falling at the same time?
She only turn the timers over when the sand has all fallen to the bottom in one of them.

Hint 1

If you simply turn over the times whenever tha sand reaches the bottom, it would take fifteen minutes for them both to finish at the same time.
But you can make it happen much faster than that.

Hint 2

When one of the timers has finished, you should turn overe both of them at the same time.

Hint 3

Figure out the right moment to turn over both of them at the same time.

Super Hint

You should turn them both over at the first opportunity.

Answer for 'Sleeping Sand'

The answer is '6'.