I have two tv shows on the phone right now and it's 1 hour and 37 minutes total time and 1.09 GB. So think of every hour of video around 700mb. So 3.5 hours would be around 2.5 GB. That the rough estimate I'd use. This may be different for movies however.
It really depends on the quality you want I used Highest Quality and these are my sizes. If I chose Normal Quality maybe I would have had a bigger decrease in size. I believe ImTOO compresses the videos also though but to a point where you don't lose quality