It's the definition of "a while" that's the moving target. There are a ton a moving pieces involving mandatory retirements, planned aircraft deliveries, the economy in general, and events way outside the control of UA.

If UA keeps hiring ~100/mo between now and next summer (the current plan) there will be lots of movement and time on reserve will be short, perhaps a a couple months. It's likely that this will occur, but there are no assurances that it will happen.

Hope for the best, plan for the worst.