Box making

Damn, you must go through a few boxes of screws on a single project. Rember it's the glue holding the box, the screws are used to clamp the joints while they dry.

I think every 3-5 inches would be adequate

Liquid nails rocks, some kind of caulk on the inside of all the joints.  The 2 inch screws are enough every 3-5 like dwarren said.  Just to give you an idea of the weight capability, the cabinets in your house are held up with only a few 3 inchers and they hold up tons of weight well.  I have some shelving in my garage with 6 2 inch screws in the studs and theres at least 300 lbs of junk on them.  also when you use screws, use a countersink to make them sit flush or just under the level of the material.  That way if you want to upholster the box with something you don't have bumps everywhere.

you have to use good screws so they thight while the glue gets dry ....and you have to leave them there!  once the glue is dry ...it is easy for the MDF to get broken because its physicall properties...it is a hard material but at the same time it is to easy to break it ...

and the screws are not expensive.. I got 80 screws for near to 3 dollars at home depot!

Use a Jasper Jig on your router.  Also, if you use poly glue make sure you pre-wet the edges of the MDF so it won't soak up all the glue.

In general, my opinion is if you use screws instead of clamps, in a well-made enclosure you should be able to remove them without changing the integrity of the enclosure.  If not, it's not well glued or braced.