Microsoft Office needs to be activated after 30 days. It can be rearmed 5 times, every rearm gives you additionally 30 days to activate.
You can not rearm anymore once Office is activated (not even if it was activated using a trial key).
If you always rearm it on the last day before it has to be activated that makes a total evaluation time of 6 * 30 = 180 days.
Although the rearm feature was made for corporate users, it cannot be illegal to use it since it's an official feature.
This tool only works with RTM (14.0.4760.1000) - or later - versions of Microsoft Office 2010! It does not work with "Click-To-Run" versions which are actually just virtualized
trial versions. It also seems not to work with 60 days trial versions!
More information from program website