Microsoft Office 2013 can be pricey, though we of course advocate purchasing it should you need to use it regularly. However, if you're thinking about using it to give it a test spin or need to use it a little beyond 30 days, How-To Geek has an answer for you.
PCWorld brings us this helpful hack, which will walk you through renewing the preset 30-day trial period originally set up for you by Microsoft. You can do it up to five more times, which can in theory provide up to six months of Microsoft Office 2013. It's as simple as opening a command window and running a specific utility. The utility must be ran every 30 days for the trick to work, but it's a small price to pay to have to remember compared to buying the software, especially if you're not sure you want to commit.
The full instructional how-to is over here. Let us know how it works if you decide to give it a spin.