I should think that you created more than you could comfortably use in the life time if the OS.
Assuming that you install a theme and leave it for 1 month, (which is a fairly short time)
60 themes are going to last 5 years.
to put that into perspective, If you made those themes for Vista on the day of it's release (jan 2007) you still wouldn't be through using them, whilst the OS has been superceeded twice.
The OS would have reached the point where it stopped being sold, and stopped even being supported, Windows lifecycle fact sheet - Microsoft Windowsand you'd still be rotating your themes trying to use them all...
Alternatively I guess that you could rotate them each and everyday, that way your computer would always seem new and a little unfamiliar.