How often should clay be replaced?

[iono11200]

I used Adam's Visco clay last spring on both of our new vehicles. I suppose I'd say I had good results with it, although it is my very first experience with clay. As I eagerly await spring I'm beginning to stock up on my detailing supplies. To those with past experience, how often should my detailing clay be replaced? 

[LFairbanks]

When you can't fold it anymore and make the clay clean.  If that makes sense.  The number of vehicles you can get out of a bar are dependent on the size of the vehicle and how bad they were.  

[Rich]

If it starts getting hard, or if you find dirt that you can feel when you're folding it. Or when it's too small to use any more.  Any of those will justify replacing it.  

[hemi1300]

When u drop it on the garage floor lol

[chops1sc]

I look for large pieces of debris and severe discoloration. I use a different brand of clay and I buy in bulk so I can ensure that I have a fresh bar on deck.