The figure I use for patients having surgery now is 10-20% in their lifetime. Yag laser if needed is quick, simple, painless and low risk.
JCH MD
The rate of PCO has been dropping over time due to improved lens designs, and I hadn't seen a good current overall figure, different sites give different estimates. Some sites suggest it is about 20%:
http://www.allaboutvision.com/conditions/cataract-complications.htm
This NHS site says less than 10%:
http://www.nhs.uk/Conditions/cataract-surgery/Pages/risks.aspx
The rate depends largely on the lens model, so for some lenses you may be able to find studies indicating the likely PCO risk.