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...