Depends on how you do it.
The easy way, just slide the blocks in, (not necessarilly right) maybe an hour.
The right way, disassemble spring pack, install block on top of spring, then the wedge on top of block, bolt everything back together is gonna take longer. Depends on if you have everything you need to do it this way.
C-clamp, new modified bolt to hold spring pack together.
If you go the easy route, the rear end winds up getting pushed forward. This will cuase the wheels to not sit centered in the wheelwells and cause it to look dorky. 1" will create a less noticeable dorky look than 2" will.
Like this: