When to replace turntable belt?

When a turntable belt stretches out, slips, or needs to be replaced, you will primarily hear a severe distortion in the pitch and speed of the music, universally known in audio as "wow and flutter".

Because the rubber loses its elasticity over time, it fails to spin the platter at a constant speed. Here is exactly what you will hear out of your speakers, as well as the mechanical sounds the machine itself might make.

Audio Symptoms (Through the Speakers)

• Wavering or Warbling Pitch: Sustained musical notes—especially piano keys, acoustic guitar chords, or long vocal notes—will sound like they are rapidly going out of tune or "warbling".
• The "Drunk" or Dragging Effect: The music will sound sluggish, heavy, or lower in pitch than it should be. This happens because a stretched belt slips on the motor pulley, causing the record to spin slower than the required 33 1/3 or 45 RPM.
• Sudden Speed Jumps: You may hear the music randomly speed up for a second and then aggressively slow back down as the failing belt desperately tries to catch its grip.

+1 @mofimadness 

Wear depends on the belt’s material as higher end TT often use better belts 

Do we have to regularly measure the rpm variance? I bet the belt wear would be revealed by the rpm stability.

@mofimadness Well done.  @swong61 belts don’t all behave the same way.  You might be right for some.  Usually the belt will stretch and what mofimadness has outlined are things to listen for.  I will add that when the time comes it is not a subtle change, but more of a suddenly things don’t sound right kind of thing.

It can also depend on the belt material and where you live. E.g., if the belt is made of a rubber compound and you live in an area with low humidity, the belt will likely need to be replaced sooner, since the material can dry out. Information as provided by the distributor of my Acoustic Signature turntable, where the belt is supposed to last 5 years on average.