My '06 G6 has an airplane-like sound that comes from the passenger front end.
I already know it's a wheel bearing, but how urgent is this kind of repair?
I'm concerned about fuel economy and safety.
Pretty urgent. Less urgent than say, having no friction material left on your brakes but way more urgent than any regular servicing.
Driving around with a dying bearing will hurt both economy and safety but mostly safety. If the wheel is not a driven one (e.g. if it's the front wheel and your car is rear wheel drive) it's possible for the hub (and wheel) to come off completely, though this is only likely on a wheel with a drum brake as a disc brake caliper would attempt to retain the hub & wheel via the disc. The bearing could also seize and cause a loss of control by locking the wheel.
A bearing failure or seizure has the potential to damage other more expensive components that are close to it, like the hub, so leaving it can be a false economy.
Get the wheel bearing replaced at the earliest opportunity and don't drive the car unless you need to as the miles are ticking away to the point where the bearing fails completely. When driving, avoid super high speed (like 70mph+) and hard cornering as those 2 things put the most load on the wheel bearings.