For 48 hour game jams spend the first 4 hours building the core mechanics and the next 44 on polish.
Credit to Rami Ismail. The rule states, for a 48 hour game jam, you should spend only the first 4 hours on concept and programming core gameplay. The next 44 hours should be spent on polish. This is to make sure you have enough time to put out a polished game in the time available.
XCOM: Enemy Unknown