If you are concerned about this, you need to start tracking the size of your database. You can then plot the growth on a graph and estimate the day you will run out of space.
You could set up a job to get this information from the system tables and then report of of this information.
I like this system.
http://www.sqlmag.com/Articles/ArticleID/95385/pg/3/3.html
Check out the other articles by this guy.