When is it time to meditate and when is it time to act? As a Mahayana follower I keep growing my wish to help all beings. To accomplish this, I have developed the wish to become a Buddha. As I meditate and follow the teachings, I really wish to help all beings to be free from suffering and want them to be happy, and I really can feel by my own experience that I can make them happy by practicing.

Although sometimes I also have the feeling I should help in other ways. Like giving them food or even doing some volunteer work to help them to be free from suffering, at least for a short period of time.

When is the time to meditate, and when is the time to help others (and of course by doing this wanting them to be free from suffering)?
I think there is a certain tendency in the west to take the Bodhisattva vow as mandate for humanitarian action.  The reams of material that have been written on socially engaged Buddhism are pretty exemplary of that.  I personally don't think there is anything wrong with working to make someone suffer a little less.  I also think that proponents of this kind of engaged Buddhism are on some pretty solid theological ground.  Helping others is an essential part of anyone's practice.  It's no accident that the first paramita (in all traditions!) is generosity.  

I think I need to qualify this though.  No matter how much effort you exert, you aren't going to end suffering in the world.  No government plan, social service, or movement can accomplish that.  It's impossible.  To even think along those lines is completely contrary to Buddhist teaching.  The only surefire means we have of ending suffering is through our own liberation.  It is imperative that we don't lose sight of that.  So many people get caught up in outward directed effort that they lose touch with their individual practice.  One wonders how truly useful they can be as Buddha-beings when their best weapons in the fight against suffering - their loving-kindness, compassion, patience, and generosity - aren't sharpened by daily practice! 

It's always time to help others and if you practice Buddhism sincerely, you will always be doing exactly that.  The hours that you sit benefit all sentient beings in ways just as powerful as any hour you spend working at a soup kitchen.  If you feel called to do something more tangible than just sitting, that's fine.  It's even recommended lest you begin to develop an indulgent attachment to the bliss of concentration.  Just don't let outward efforts distract you from your own liberation.