below are the steps  to extend  trail period

Note: This hack will work after every days of use, so try these commands 2 times just some days before your 30 days trail is going to end.

When installation is complete, open Command Prompt.

Type any of the three commands below:
sysprep /generalize
slmgr.vbs –rearm
rundll32 slc.dll,SLReArmWindows

This is how you can get 90 = 30+60 days of Windows 7 trail period.