It really depends on the quality of the source on the DVD as well as the length of the feature. I can't give you an estimate for how much space each minute takes up, but I have some examples that will hopefully help:
I initially encode using Handbrake, and for most movies I get a file that's somewhere around 1GB in size. I then run it through iSquint with it set on quality set to 'high' and it usually cuts it down to the 500/600MB mark (or if it's just set to 'standard' quality usually 400/500MB).
The initial Handbrake encode for Transformers (almost 2 and a half hours long) was around 1.2GB, then running it through iSquint got it to around 850MB.
Compare that with A Hard Day's Night, which is fully black-and-white, standard definition and 80 minutes long. Initially after putting it through Handbrake it was approximately 800MB, then re-encoding it in iSquint cut it down to 400MB.
I know it isn't exactly what you were asking for, but I hope in offering a view of the sizes for a high definition movie that's incredibly long compared to a standard definition, low-quality film, that this has been able to help you.