Assuming that the patient drive is in pretty rough shape, the time to clone could take a week or two. But, usually a day or two. Then assume it takes another day or two to recover the data. The clone sits at least 2 weeks before being wiped and tested before being put back into circulation for the next job.
So, a drive may be powered on 300-400 hours in a 720 hour month (30 x 24). At that, it is not taxed at all, as the write pattern is in a slow steady motion, not requiring a lot of seeks. In the world of data recovery, I'd rather have 10 cheap drives for cloning than 5 expensive ones. Of course, if you only do 1 or 2 jobs a month, it won't matter what you have as you will have plenty of time to wipe and test your drive and get it back into circulation before the next project needs it.