By Default Activation Period is set to 30 Days But Legally you can make this 120 Days for Windows7 & Windows Vista too. You have to Simply run slmgr -rearm to get another 30 days at an administrative level command prompt. You can do this a maximum of 3 times before Microsoft blocks further extensions.
Learn How To Extend Windows7 & Windows Vista Activation Period to120 Days...
# Click on the Start Button and type Command.
# The Command Prompt shortcut will now show up in your start panel search results.
Right click on the shortcut and select Run as Administrator.
# After Command Prompt loads, type in slmgr.vbs -rearm and press Enter.
# Reboot your PC.