Here's an actual data from my Hauppage 2250 on HD recording...
An hour of HD TV (1080i) recorded is ~45Gbyte.
I recorded Superbowl, Grammy Awards Night, FIFIA Finals... The recorded files is between ~25Gbyte to ~27Gbyte.
I use one of my PC as my DVR/PVR..i Allocated 2Terabyte space for it so don't worry about space.