Transcode time will increase with compression level. The average transcode time for a 2-hr movie at 80% video quality is around 39 min with CloneDVD (600 MHz PIII). If you are not compressing the movie, then the processing time should be close to the read speed of the DVD-ROM (usually 10 to 12 min for 2-hour DL movie).
Since you are copying the ENTIRE movie, your video quality should be 60%, plus or minus 5. I would expect about 25 min average with that setup.
If you must have speed, then use AnyDVD with Nero Recode. No need to rip with DVD Decrypter. You should be ready to burn in less than 20 min with this combo.