Will soldiering be required? How long would it take to take one switch off?
First, you have to remove the case and take out the plate/pcb assembly. Then simply desolder the switch you want to remove, and pop it out of the plate. Doesn't take long, but the tricky part with a Filco is that the holes are double sided, plated all the way through. This sometimes prevents you from getting all the solder out of the hole, especially if you are using braid. You really need a solder sucker for the job. If you try, and there is still solder left in the hole, just add more solder and try again.
So, in short, the time it takes you will just depend. On your available tools and soldering skill, mostly.