About one to two hours at the most. You need to disable write cache manually (at least that was what the Procedure Generator stated last year ), unscrew several screws, remove cables and the SPS, which is heavy and needs to be done carefully, because of all the fibres you normally would encounter in a rack with a Clariion installed. After removing the old SPS, you can slide in the new one and reverse everything: so tighten the screws and enable the write cache again. If you take unpacking and packing into account it's 2 hours max I guess.
Are you replacing a power supply for the Disk Processor Enclosure or a Standby Power Supply (which is the process that RRR detailed) ?
They are different process, but both are hot swappable and will be done with all your hosts online. Worst case scenario is that you will have to disable write caching during the replacement which will impact the array performance. So it is best to do this after business hours if you have applications that are sensitive to i/o performance.