Roasting meat at low temperatures
I can't comment on how long it takes but I can comment on the difference I've noticed between each method.  To start out I could never explain the how and why better than J. Kenji Lopez-Alt, over at The Food Lab, so here is what he has to say about high vs low temp roasting:
    http://www.seriouseats.com/2009/12/the-food-lab-how-to-cook-roast-a-perfect-prime-rib.html

If you still want to read here is my take on it.

To me the results were almost identical for the final outcome of the meat.  However the difficulty in achieving the right level of done-ness is what differed the most.  With the low temperature method, you have a fairly large window of time where the meat is in your 118-120F range.  Contrast this to the high temperature method where it is easy to miss by only a few minutes and quickly overshoot your correct temperature.

Another benefit of the low temperature method is that you can pull the roast from the oven, and let it rest while you finish preparing the rest of the meal (in case timing everything isn't your forte).  Then when it comes time to serve, you put the roast back into a screaming hot oven, you get the nice brown crust on the outside, warm the meat up again, and can immediately carve the roast right out of the oven since the interior has already had a chance to rest.