As kind of standard operating procedure, I have my Win XP 32bit AutoCAD user base defragment their HDD's often. I've found this especially helpful with Civil 3D users, but for any really, who use heavy raster, as these larger files tend to destroy a hard drive and slow performance.
Recently, I encountered my first user with an SSD, which was fragmented over 25%. Although I'm convinced this would slow performance with a conventional HDD, I'm not sure how to advise with an SSD. The end user is experiencing a lot of erros and slow downs that historically, I'd attribute to a fragmented drive.
You will see some degradition of an SSD over time, however it should not affect preformance all that much, one key thing with SSD drives is that onece you get close to the max. disk space they will start to slow down, just as a regulars HD would. So if you seeing performance issues that's probably the reason. Some of this is due to Widows use of a Virtual Disk, and the size it is set too.
As a general rule you really should keep about 25% of a SSD free. I have been using my 120gb SSD for about 2- 1/2yrs now, will little performance issue.
Also, NEVER DEFRAG an SSD it should have told you something along those lines, within the documentation when you purchased it.
Just as the others have stated, you never defragment a SSD. There's no need to do that and it can cause the SSD to become corrupt. Because there are no moving parts in a SSD, it doesn't matter where on the SSD parts of a file are. All of the sectors can be read almost simultaneously, so even if a file isn't allocated in the same area it doesn't matter.
If you're having space issues, I'd recommend moving the swap file to a HDD. That can free up a lot of space, plus increase the longevity of the SSD drive as it won't be written to as often.
Infrastructure Design Suite Premium 2014 Civil 3D 2014 x64 Windows 7 64-bit 6GB RAM