Hours will be all over the map. Chilton, Alldata and the dealers all have different labor estimates, and even then, there is a different scale for what they'll get paid for warranty work from the factory, what they sell at retail, what they pull out of their butt when people ask, etc. Hourly shop rates are all over the map as well. IMO, just forget hours and focus on total cost and whether you feel the shop does good work.
IMO, a motor is a wknd job. 1 day to pop it out and clean and replace exposed, damaged and related parts, another day to get the new motor back in and tested. I've never done a Sprinter in particular, but several others. Maybe 20 hours.
However - rebuilds are dicey. The conditions and quality control around your motor when built new and a rebuild are night and day. I'd have more faith in the above 2 posters than a shop that does motor du jour and hires a bunch of min wagers to get all Frankenstein with other people's trashed out motor scraps. You know your motor and its issues. You have running proof of what is and isn't working. You have its history, you know if it's ever been overheated, etc. Consider having someone who has done a min of 5-10 of that exact motor tear your motor down and rebuild it.
Otherwise, IMO, and my experience, unless you have the skills and experience to take charge of things and do it yourself and get it right, or are willing to search for the rare bird who can handle it for you - it's a huge waste of time and money - just get a new truck. It's just too complicated of a job to expect anyone but the most focused of specialists to get right.