It depends a lot on what you actually download.
A full film in the format you normally see on the net (and which can be downloaded ILLEGALLY) is usually around 750MB. So you could get approx 6 or 7 of those each month.
MP3 music tracks, stored in the medium quality of 128kbps are usually around 3.5->5MB per track. So you could get between 1000 - 1400 of these.
But remember that general surfing, reading various webstites, sending emails, watching streaming video on likes of YouTube, or listening to streaming rado channels also uses up your allowance.
5GB is a decent amount of stuff. Unless you plan to download loads of films or music it should cover you ok.