Drivetrain wear -- questions on chain elongation and skipping
According to Park Tools:


  A worn chain shifts poorly and wears sprockets at an accelerated rate.
  The CC-3.2 is a go/no-go gauge designed to accurately indicate when a
  chain reaches .5% and .75%, the points at which most chain
  manufacturers suggest replacement. For 9 and 10-speed chains, replace
  chain just as the gauge fits the 0.75% side fits flat into the chain.
  For 11 and 12-speed chain, replace as the 0.5% side fits. The CC-3.2
  is long, accurate and features permanent measurement markings.


The 0.5% - 0.75% answers when you should replace the chain but does not answer the max stretch before skipping.   

Sheldon Brown's site ( http://www.sheldonbrown.com/brandt/chain-care.html ) indicates things get dramatically worse once the elongation reaches 1.0% because at that point it is every other link that is load bearing... so I would assume >= 1.0% is the answer with the variance being attributed to the condition of the cog.  Checkout the link and you will learn more than you can imaging on chain-cog engagement.


  The best way to determine whether a chain is worn is by measuring its length. A new half inch pitch chain will have a pin at exactly every
  half inch. As the pins and sleeves wear, this spacing increases,
  concentrating more load on the last tooth of engagement as the chain
  rolls off the sprocket, thus changing the tooth profile. When chain
  pitch grows over one half percent, it is time for a new chain. At one
  percent, sprocket wear progresses rapidly because this length change
  occurs only between pin and sleeve so that it is concentrated on every
  second pitch; the pitch of the inner link containing the rollers
  remaining constant.