The stats don't really matter, because the market is designed to equilibrate the price: it costs 7.5 million to mine 7.5 million's worth (in expectation, net present value and all that), and always will. If you're worried about reducing your variance you can join a pool rather than mining directly.

But over what time period?

At a guess we'd be talking 6 months - 2 years - my vague memory/impression is that's roughly the timescale on which mining hardware goes from new to worthless. If you're more vertically integrated in terms of buying datacenters/chip fabs/what-have-you then I'd imagine longer.

There was an article posted a few weeks ago about a large Chinese mine that was doing $200K/day.

Sure, and that's a lot, I'm just wondering if there's anything approaching a investment->return timescale.

Not that it really matters! It's not like I have a few mil hanging around looking for something to do...

You can mine in a pool that distributes the pool's winnings proportionally to hashpower to its members. You don't need to actually mine any blocks on your own.