Rim Wear - How do I Know When It's Excessive?

[Eugbug]

How can I tell whether my rims are worn to the extent that they're liable to split from tyre pressure? They've had about 7000 miles of travel but the wear depth is probably 1/4 mm. It could be more, but since the pads have worn all over the width of the rim, there's no unworn section to compare it to. I could measure the thickness with a micrometer.