Data download allowance of 250 mbs per month how many hours does this allow?

First off, contact your provider to see if this is 250 megabytes or megabits. Make sure it's bytes to avoid getting ripped off..

Anyway, it depends on what you do. An average 2 hour long movie is about 900 megabytes. If you commonly use the internet, you could use this up in less than a day.

I would, personally, choose a better plan, depending on how often you use the internet.