How long before Trump leaves Syria?

It is hard to know for sure, but the options are clear.

On the one hand, Trump could escalate the conflict - likely on the side of Turkey and with implicit allegiance from Syria, Iran and Russia - in removing ISIS from Syria completely. It would make sense for Turkish and American forces to attack northern Syria across Aleppo and Raqqa provinces, to proceed in building safe zones in that region taken from ISIS. After the establishment of such safe zones, it would be likely Trump would withdraw from Syria.

Meanwhile Assad, Russia and Iran would move from the central region of Syria to secure Homs and Deir-Ez Zor provinces from ISIS. This cooperation to destroy ISIS could be readily achieved with Turkey already allied with Russia in defeating ISIS.

On the other hand, Trump could leave Syria altogether and leave Russia, Turkey, Iran and Syria to fight ISIS. This would make the tangled web less complicated, and perhaps even a deal could be struck between America and Turkey: that Turkey's efforts in Syria would pay for a certain amount of their NATO budget.

Trump certainly does not need to stay in Syria, and in my own opinion, to do so would be economically foolish. With Afghanistan, Iraq, Libya, Yemen, Pakistan and Somalia on America's list of countries they are bombing, Syria is just too financially costly a campaign to maintain. Trump exiting from fighting ISIS in Syria would force other players on the ground to take more of the mantle, and consume valuable resources of other players to their own economic detriment - in particular Iran.

It makes more sense for America to focus on fighting ISIS in Iraq and the Taliban in Afghanistan - the coalitions fighting in Syria have already shown an ability to make their own headway in ending the conflict without Washington.