The time it takes to do a full interior detail varies. The time frame I commonly see for interiors w/ carpet extraction is 1 1/2 to 3 hours. That includes equipment setup and break down. Just depends on the size of the vehicle, if it's got leather, how trashed it is, etc.
A carpet extractor is a LOT better than a Wet/Dry vac. If you are doing it professionally, then you'll want to use a carpet extractor.
The carpet extractor has a lot more suction. Also, it is made to rinse the carpet out with water. A wet/dry vac would just suck the foam, etc. up. You can use a spray bottle with water to rinse the carpet out but it would take a lot of extra time. You can also see how dark brown the water in the extractor is. Just think you'd be leaving all the dirt in the carpet and just pushing it down into the padding and spreading it around.
If you ever want to do serious odor removal, then you'll need a carpet extractor. Same for cleaning stains out of the back of some seats. You'll want an extractor.
I've done it in the past with a wet/dry vac and now I have my carpet extractor. You'll see a big difference in how easy it is to clean out really bad interiors and the customer will notice how much quicker the interior dries.
Also, leaving soap residue in carpet fibers can actually cause stains to set in the carpet and be more difficult to remove in the future.