It's almost impossible to determine what your data usage will be with netflix or any other online source. Std def movies, HD movies, 3D stuff, all take vastly different BW. And even for a give format, BW needs can vary base on the amount of compression that can be used on the source based on the original video. still/slow motion takes less BW than sports for example.
Netflix web site will let you choose max data rate for downloads, and can give you ranges for data rates/hr for various formats. Suffice it to say pick one (lowest BW and quality, which is comparable to old NTSC broadcast video) and you can end up watching all month on 5GB. Pick another, such as HD 3D and you can use 5GB in a few hours of video or less.