How long should I meditate?
There is no given rule.
Ultimately, you should be the master of your practices.
It is your life, your desires, your wishes.
Meditate ten hours in a row or a few seconds if this is what you wish.
Your spirit gets the message anyway.
As a general guide line, I would say give rhythm and energize by short meditations rather than lengthy "concentration marathons".
Ultimately, the fun and enjoyment of the moment is what makes you come back to it.
A practice based purely on discipline will fade away.