Hi,
There are some variables based on your current deployment and if you have 3rd party plug-ins.
But there is no downtime to the VM's only Vcenter and that is minimal at best. Once the Vcenter instance is upgraded then you can upgrade each host to the new vSphere, now this does cause downtime for each host once Vcenter itself is upgraded. if you have vMotion setup you can just move the VM;s off 1 host at a time while you upgrade the host.
Hope that helps
