Which distro and version? Linux 2.6.33 onward has supported TRIM.
Lifetime expentency has to do with your particular usage of the swap partition and OS handling of it. In Windows 7, reads outnumber writes by 40:1 on average (http://blogs.msdn.com/b/e7/archive/2009/05/05/support-and-q-a-for-solid-state-drives-and.aspx)
However, it your workload is exceeding the installed RAM then swap will obviously be more heavily utilized.
Basically, you may want to do some benchmarking and monitoring if you want to be sure.