The ESX/ESXi 60-day evaluation period begins to count down immediately after the first time you power on the ESX/ESXi machine.

The 60-day evaluation count down starts even if the host is licensed and you are not using evaluation mode. For example, suppose you decide 10 days after the first power-on to switch from licensed mode to evaluation mode. Only 50 days remain of the evaluation period. Sixty days after the first power-on, it is too late to switch to evaluation mode because zero days remain of the evaluation period. During the evaluation period, if you switch the ESX/ESXi machine from evaluation mode to licensed mode, the evaluation timer does not stop counting down.

To prevent losing the availability of the evaluation mode, VMware recommends that before (or shortly after) you power on your ESX/ESXi machine for the first time, decide whether you want to use evaluation mode. One advantage of using evaluation mode is that it offers full feature functionality, which lets you try features that you might not have yet without paying additional license costs.