How often should a PC be defraged? :
When you experience slow seek and slow application startup times compared to when you knew the system was fully defragged. You know, those times when you say "that's taking ages to.....". Or when you reach 10% fragmentation on your HDD that houses your OS.
Others after every install/uninstall of software: (edit - misread that but my answer stands)
No, just NO! Fragmentation is caused by uninstalling and reinstalling software. You should always defrag before or after large amounts of data are added or removed from your HDD.
Why does the standard defrag program from windows take hours and hours:
Because it's sh1t. Simples.
You are advised to use a dedicated 3rd party program that can be initiated when you want and can defrag your HDD by these means: By Name, By Access Date, By Size etc.....I recommend O&O Defrag. This program manages my system and keeps my fragmentation below 0.15%.
A last word on Defragging. Occasionally (your choice) it is advisable to disable your Swap File (aka Virtual RAM), reboot and then defrag. This file occupies a large section on your HDD and can easily become spread out over the sectors on any HDD.
The aim of defragging is to move and rearrange all your programs so that their data sets are continuous on your HDD sectors, not having them arranged together means that your system is searching for different data sets in different areas of your HDD. Not optimal.