Depends. If they beat Trump in 2020 and the economy goes into recession before 2021 or Trump is re elected, then the next President serves 2. If Trump loses despite a strong economy, then the next President is likely a one termer.
This. It all depends on whether the stock market correction comes.
There's also the 1920's scenario-the boom continues straight into 2025 or 2026 and gets Pence or Haley elected, but the fallout from a crash if things have stayed good that long would all but ensure 3+ Dem terms in a row after that. A polite version of Trump would probably have 2/3rds approval in this economy, given how Hogan and Baker are polling in extremely blue states.
Right now, I'm actually thinking 1-termer, probably the 1920's scenario, particularly if Dems get the House by a wide margin and hold it through the 2020's. Trump and a Dem House would likely do massive rural-focused stimulus if the economy gets wobbly after 2019.