depends on what you have. If you got a lift, it will be much easier and quicker. I spent a good 4 hours to complete the task mainly because I had to lay under the truck and turn those rusty bolts with limited space.
From loosening the bolts to taking the whole shaft out took me about 1 hours, without using any air tools. The U-joint was stuck and took me another 30 minutes to figure out the long pry bar was my best friend. A 2 prong puller is needed to pull the bearing out of the shaft.
While the shaft is out, check the rear output shaft seal on your transfer case, see if the seal is in good shape. I didn't and after I put everything back, there was a slow leak from that seal. I ended up ordering the seal and had to remove the shaft again to install the seal.

If you have a trusty mechanic, it will probably take 1-1.5 hour to complete the whole job for you.

__________________
98 ML320 (gone but not forgotten)