The engine should be fine. Assuming it has a CVT automatic transmission that's the weakest part of the car, probably good for 100K to 150K miles or so. Subaru CVTs built from 2015-on are improved over earlier models.

How long should you keep it? Well, as long as it does what you need it to do. And, keep it as long as you like it, and as long (at least in my estimation) as it's economically feasible to do so. That's always a contextual cost/benefit equation.

Cars are losing economic propositions, with various degrees of loss. The purpose of good maintenance is to spend a bit more, when needed, to save considerably more, later. Eventually, as we know, every car reaches the point of NEFR (Not Economically Feasible for Repair). It's in extending when that happens that we can exercise some control, both in our choice of vehicle and how we take care of it. 

It's like humans: Enjoy your vehicle, while it's young. Take reasonable care of it as it ages. And let it go, when the pleasure or the economic equation decisively changes. 

Keep it until 100k miles and take care of it.

If you can get a decent amout around 100k miles, sell it. If not, keep it a while more.

The weakest part of the vehicle is the CVT which goes out around 100k miles so you might want to sell it before the warranty is gone.

All in all they are pretty well made you shouldn't have major issues before the 100k miles mark if you take care of it.