Its just a hidden feature like Windows 7 have too. After the extension period is over, you have to buy.
Microsoft Office 2010 needs to be activated after 30 days. It can be rearmed 5 times, every rearm gives you additionally 30 days to activate.
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.
You can use this tool to rearm Office 2010 5 times. Just re-apply before the 30 days trial expires.
This tool only works with RTM (14.0.4760.1000) builds of Microsoft Office 2010! Click-to-run versions (e.g. 14.0.5114.5002 and 14.0.5113.5000) seem not to work – which makes sense because those are just virtualized versions.