The vSphere Client doesn't expire, it's eiterh the ESXi host's and/or the vCenter Server's license wich can expire. Anyway, assuming you are running a single standalone ESXi host, the free Hypervisor license might be what you need. In this case register for the free license key at http://www.vmware.com/go/get-free-esxi and assign the key to your host following the steps in "Licensing a standalone ESXi host" at http://kb.vmware.com/kb/2014295
André