I have changed the bearings in almost all of my motors at least once. They usually go well over 200 flights, regardless of the way the motor is mounted. Heavy props and spinners tend to wear bearings out faster.

You will notice excessive play at the prop (either/both fore and aft and side to side play) or noise...typically a kind of rattling sound and vibration. Bearings are easy to change (note: e-flite glues in the bearings with what appears to be cyanoacrylate...a gentle tap is enough to free them). I use a punch made of 1/8" music wire to catch the lip of the bearing, moving it around the circumference of the bearing until it pops out. 

Bearings are available from a number of sources. BOCA bearings in Florida is fast and reliable. RCBearings.com have a good inventory as well, but I have not been impressed with the quality of their bearings. Innov8tiveDesigns.com (suppliers of scorpion and cobra motors) stock bearings for all the motors they sell at very reasonable prices.

Hope this helps.

Bob

Here's a video on YouTube that shows how to change bearings on a Scorpion motor.  Pretty much the same for most motors.

Just be aware of the big three and a half ways of retaining shafts in motors.  Get it wrong and you'll damage something trying to take it apart.  I think I've seen all three (I'm not about the 1/2) on our motors, but I have NO CLUE about which ones are most used: I just know what I've found with 30 years of working with similar things in industry.

• Press fit.  Done if the motor is really fancy or really cheap, this depends on the friction between the shaft and the housing to keep the shaft in place.  The Scorpion motor in the video appears to be a press fit backed up by the collet with set screws.
• Shrink fit (this is the 1/2).  Done if the motor is really fancy.  You chill the shaft, heat the housing, then assemble.  When things reach equal temperatures the fit is TIGHT.  You may or may not damage a shrunk-fit motor trying to press the shaft out.  With an aluminum housing and a steel shaft you can usually loosen things up by heating everything, but you have to be aware of what the windings can stand -- and winding insulation temperature ratings vary!
• Set screws.  They'll either be in a collet or in the motor housing.  These often fit into flats that are milled in the shaft.  Try hard enough to press a shaft out without removing the screws and you'll damage shaft, housing, or both.
• Circlips (AKA "E-clips" by marketing people, or "Jesus clips" by anyone who's ever pried one off, heard it go "spiiiiing!" into some dark corner of the shop, and said "Jesus!  Where'd it go?").  Leave the circlip on, press hard on the shaft, and the only way something will move is if metal bends or breaks