Ubuntu releases that are not LTS releases are supported for exactly 18 months. That is 1 year and 3 months if you want it in years and months form. Today is Ubuntu 11.04's end of life. If you are using this version of Ubuntu, I recommend you upgrade to the 12.o4 LTS release.
As for direct upgrades instead of a fresh install, you should upgrade to 11.10, then go to 12.04 LTS. However, if you want the best experience you should always go with a clean install to make sure that you don't have left over "gunk" on your system with left over packages and backups of files and temporary files, etc.
So...so long 11.04. We'll miss ya -- or not? I'll go ahead and embed the 11.04 default wallpaper below if you guys want to grab it...maybe as a tribute.
If you are wondering what Ubuntu release I'm running, I am running 12.04 LTS with Unity2D and XFCE as the desktops I use.