Normally we do not recommend replacing single rigging wires.  They are generally much better replaced symetrically in pairs or whole groups. 

Pilots often worry about replacing rigging wires before they have got their 'full monies worth' from the old wires.  Clearly the only way to get 100% of the life is to wait until the wire fails.  While this could save you money on wires you could end up paying a very high price in other ways.

A far better way to view wires is as something you want to replace before you have got your full monies worth.  This is more expensive but does make for a safer hang glider and less worry in flight.

Clearly judging when a wire should be replaced is a difficult thing to do.  If they are corroded, kinked or damaged in an obvious way clearly they should be replaced straight away.  If apparently in good conditon theyn the age of the wire should be looked at and the BHPA recommendation for annual replacement born in mind.

Don't be too mean with wires.  They are not expensive, especially when the cost of failure is considered.