I constantly look under the vehicles for leaks. None have been present - so I've grown accustomed to checking the oldest vehicle every three weeks (approx 700 miles). With 170K on the odometer, I usually find it a half-quart low.
The rest of my vehicles are younger & driven less miles, but since I'm already outside checking the oldest one, I raise the hood on the other vehicles too. Usually, all that's needed there is a refill of windshield washer fluid.
On older high mileage vehicles I check every fill up. On newer vehciles I check about every 2,000 miles. If you have oil consumption issues then checking at every gas fill up is a good insurance policy against running low on oil.
Totally vechile dependant. After an OCI or two I can tell how much oil my car's burn. I check it dpeending on how fast each particular car burns the stuff.
On my Saturn for instance, I'm dumping in a quart every tank and a half of gas :/
I do "motor stables" every other Sunday. This involves routine operator maintenance on all vehicles and other engine-powered equipment and includes checking all fluids, tire pressures, and looking for anything out of sorts. Of course, these checks don't take the place of scheduled maintenance. I keep logbooks on some equipment. Sounds a bit obsessive, but I just call it "organized."
I check oil and tire pressures every weekend unless I forget or am out of town.
Mostly to get out of the house, but I do like to keep tabs on things. This routine has discovered 2 'slow leak' tires that I was able to repair before they got trashed. Never had 'oil level' issues though thankfully.
BTW - Good work in picking a subject title that everyone can agree on!