Those 48 hours are real-time rather than in-game, so unless you're planning an unadvised binge you won't get the full two days of free-ness. You can start the free time whenever you want, however, as the 48-hour countdown only begins after you launch the game for the first time. Also, if you later buy The Sims 4 then your Game Time progress does transfer over.
The Sims 4 isn't the only free rental in Origin's Game Time library. PC owners can also enjoy a free 48 hours of Titanfall and Kingdoms of Amalur: Reckoning, or an even meatier 168 hours of Battlefield 4 - otherwise known as seven days, a week, or roughly 1/52 of a year.
So, you can now try before you buy The Sims 4, but if you'd prefer to read before you try then check out our review. According to Alexander Sliwinski, if you're new to the series then The Sims 4 is a "great place to start and explore the possibilities," since it mostly feels like more of the same, trapping-Sims-without-a-toilet fun we've known for more than a decade.