after how long is a movie released on dvd?

It can be as quick as three months after the cinema release these days - all depends how the studio wants to cash in on the movie, globally.

so they make more money when showing it in the cinema?

Depends on the movie - some films that don't do so well in the cinemas are bigger hits on DVD/Blu-ray/TV sell-on.

ok but i think it should be released straight after its taken of the cinemas maybe this will help them aswell as us eh?

But remember, movies are rarely released simultaneously around the world - they want to maximise the box-office run for a film in all territories (which in turn increases awareness of a movie, encourages repeat visits to the cinema etc) before ramping up the disc releases.

More money would be made at the box-office if they would not charge so much money to see a movie... more people would go and see more movies if they were not so expensive imo. It costs me over £25.00 to go with my wife and two of my kids.... nevermind eats. 

Clare Newsome:

But remember, movies are rarely released simultaneously around the world - they want to maximise the box-office run for a film in all territories (which in turn increases awareness of a movie, encourages repeat visits to the cinema etc) before ramping up the disc releases.

...And in the case of Dances with Smurfs, have another crack at wringing some money out of cinemagoers before the 3D version makes its appearance on Blu-ray...

The gap between the theatrical release of a film and its home-video release has narrowed significantly in the last three decades, just as sales of DVDs have produced greater revenues than theatrical runs for more than a decade.

In the 1980s the gap between cinema and home video releases was often up to a year (E.T., for example took even longer). Films also ran in cinemas for longer periods. Once home video revenues exceeded the theatrical box-office, the gap between release dates narrowed, largely because it made sense to capitalise on sustained consumer interest in a film. 

As posted above, the gap between theatrical and DVD releases is now often somewhere in the region of three months.

Release the film after a lot of publicity.....then release the blu-ray & DVD with greater publicity......then release the film again in cinemas with extra 6 minutes of footage!!......then re-release the blu-ray "extended version" or Director's Cut".......and then, as if the cow hasn't been milked enough to death already.....release the 3D blu-ray version

If Disney has its way, we'll see DVD releases much earlier than the current trend.

Oddly enough, Disney - back in the days of VHS video - were incredibly precious about video releasing their classic titles. I used to work for a cinema chain called AMC, which kind of introduced the multiplex concept in the UK in 1985 and at the tail of the 80s, I recall seeing the blurb for one of the films going on VHS (as we had a video outlet too) stating that there was a limited print run. How times change!