No idea how long it'll take on a W220... But with very little experience, it took me about 8 hours from start to finish to safely remove (and mark as I'm going along) a w126 dash. Once you've taken yours out, putting the new one in right away should be easier.
Some of the things that suck really bad:
Taking the steering wheel off (and making sure you put it back on properly, so you don't end up needing a realignment). Disconnect the SRS so the airbag doesn't blow up in your face
Dropping the steering column. You'll likely have to take out the ductwork in order to do this, to have enough room to physically remove the dash after its detached
Keeping track of all the screws, rivets, bolts, etc. If you have sandwich baggies, I used those and taped the bag with the bits next to where it goes. You'll seriously have a pile of screws if you don't plan it out
You'll have to take quite a bit of the car apart to get the dash out, unless they made it way easier than the w126. A pillars, nav system, disconnecting the rubber around the main central vent and properly putting it back on
I ended up messing up on my reinstall on my old 560SEL and having an indie do it for me. He only charged me 700 for redying parts that needed it and reinstalling. Worth every cent
Wish I did that from the get go