It's overkill, these days.

I sometimes clean with some wd-40 (dampen a rag and give it a sane wipe down) and then relube with whatever I have around; PJ1, teflon, etc.

Mileage is basically whenever I think of it. The heavy-duty clean and relube, when it's been days of rain riding, or it's just gunked up, I get the brush and some Simple Green and clean it down, give it a rag wipe, then again, spray it with whatever I have.

It's a 5 minute thing. I rarely do it on a fixed schedule, and I almost never even do more than give it a casual thought while on a tour.

I don't think I see enough variance in mileage from super anal scheduling or just whenever to make it worth bothering to get picky about it.

YMMV

All the important line is locked within the O/X rings.  You're just keeping the rust off.

Cablebandet nailed it O/X just keep it clean.

If you keep it wet and grit free all is good. THe wet lube allows the grit to get grabbed by the oil and flung off. Sort of the reason a ProOiler works sooooooo well. The ProOiler will keep the O/X rings wet/lubed and allows the grit to flng off.

With the absence (OK I understand the mess chain oilers make) of a ProOiler just keep it clean and wet, replace as needed.

it seems they last about the same amount of time for me no matter how I treat them. I lube them every 1,000mi. or so and replace chain/sprockets as a unit when they are worn (start requiring frequent adjustment). I wouldn't make too much work out of it.

Never spray anything other than lube on a chain, and never use a brush or a pressure washer on it. Replace it when the links start to get stiff.