I see a dynamic wherein the GOP (both voters and officeholders) act as his firewall, until they don’t. If there’s either: (a) an economic downturn; (b) a brutal, House- and Senate-losing midterm for the GOP; or © both, then I can see the end coming quickly.
Bizarrely, the outcome of the Mueller investigation may be a less relevant factor than the above because Trump is already clearly guilty of impeachable offenses just on the information that’s publicly available to date, and because under the current set of circumstances, the GOP (both voters and officeholders) would probably find a way to rationalize even the most nightmarish Tom Clancy/James Bond revelations. So really, then, it all comes down to factors (a) and (b) above.