How long do you think a 6 will last without significant repairs?

'15 6, '06 5
Current-gen Mazda6. Nothing more expensive than an alternator, and maybe not even an alternator. Highway miles, hot weather, very little city traffic.

250k miles?
Portland, OR USA
2004 6s Wagon
This all depends on how you drive and maintain it. Adult driven (aka. reasonably) and conducting all maintenance regularly I would think well past 200,000mi