on a quad mast 4 to 5 hrs would be if everything went perfect and you've done it many times before and knew exactly how to do it.
For a first time attempt i would add a couple hr's to that.

The freelift cylinder is usually easy to do, no issues getting it out as it is with any freelift cylinder. The outer "main" cylinders are a whole different story. The manuals all suggest removing the mast and laying it down to remove or repair any part of it which i'm sure they suggest this for safety reasons.
But with that said it can be done on the truck if you have the right tools to block or chain up the mast sections after you raise it to the right point to allow the cylinder to clear all the areas so it will come out. With these quad masts you have that extra channel so there is more weight to consider when blocking or chaining up the mast sections so use a heavy chain. Of course if there are any attachments on the front those will have to be removed to lessen the weight.
If indeed you actually need to repack the cylinder completely then the rod will have to be removed so you'll need to actually remove the whole cylinder to change the seal on the rod along with the packing nut seals. In alot of cases if the upper packing nut seals are the only place it is leaking you do not need to remove the cylinder, just raise the mast, block or chain it up, disconnect the cylinder rod and use the hydraulics to lower the cylinder rod enough to get the packing nut off. Then just replace the seals in the packing nut and reassemble it in the reverse as you took it apart. (keep in mind there is a crossover tube that runs from one cylinder to one of the other ones so that will have to be removed and the fitting removed from the cylinder rod as well).
Without removing the cylinder then yes you can do it in half the time that was suggested as long as you use extreme caution in safety measures for securing the mast channels.