If that's not an option, or you received an Office 2013 trial on a new PC or downloaded it via other channels, you're typically limited to 30 days to test-drive the suite.
That's enough for some users, but to really see how it meshes with your operation, a longer trial would be better. Heck, maybe you just need some extra time to save up for the software, which isn't exactly cheap.
Fortunately, as you could with Office 2010, Microsoft allows you to "reset" your Office 2013 trial as many as five times, effectively giving you six months (more precisely, 180 days) of free usage. Although this option was really intended for IT administrators, there's no reason you can't use it with your own installed trial.
As explained by the How-To Geek, the process requires a short and simple visit to a command line. Here's the simplest way to go about it:
1. Open Explorer.
2. Click inside the address bar, then paste in the following line:
3. Click inside the address bar once again, then type in the following: OSPPREARM.EXE
4. Press Enter and you should be all set.
The key thing to remember is that you must do this every 30 days; because once your trial expires, there's no going back. So mark your calendar.
Because I'm already using the retail version of Office 2013, I haven't been able to put this to the test on my system. But based on anecdotal evidence I've read, it works. Your mileage may vary, of course.