If it is a bad bearing it may be only howling at a low rpm because once it gets going, it may warm up and settle down because it's spinning faster.
As far as urgency, that all depends. It could go that way for several miles or it could seize up within a few minutes. If it seizes up, the clutch fork could push the frozen bearing into the fingers of the pressure plate (which are spinning) and grind them down till they fly apart and jam the clutch disk which would blow it up and mess up not only the pressure plate but possibly scar up the flywheel.
Can you replace a faulty throw out bearing yourself .. ?? Of course you can.
* jack the car up or put it on a hoist
* acquire or fabricate a transmission lift/jack
* unbolt the driveshaft from the transmission
* disconnect the clutch slave cylinder
* unbolt the bell housing from the engine
* unbolt the transmission mounts from the frame
* pull the transmission off the spline
* examine the flywheel for damage
* replace the clutch disk and pressure plate (might as well since you're there)
* examine the clutch fork (lubricate if it's OK)
* replace the throw out bearing
* disconnect anything else I forgot to mention.....
* put it all back together ...... see how easy ......(NOT!)...... I've done it several times.
OR
Use the humfrz method ....... drive or toe the car to the dealer (if under warranty) or a local transmission/clutch shop.
Ask them to figure out what is wrong with your car and have them fix it .......
humfrz