at what mileage does the engine need to be replaced?

millage is not the factor,,,,, its the condition of engine ,,,weak power --smoking --missing ...that matter ,, i had a honda civic 283,578. miles when i sold it to a friend ,,,his son is driving it now at over 300,000..... i have seen other cars that were trashed -- no oil changes -- at 55,000

Typically maybe 150,000 miles but taken care of it can go 300,000+ miles but a lot less if it is neglected. If you maintain it well it can go a very long time. If you want your engine to last a long time ignore the factory service interval for oil changes and change it at least every 5,000 miles or even more often.