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The most common discount is 90% and travel is on a standby basis.
Some airlines also offer a 50% discount with a confirmed seat.
In addition to discounts offered to staff by their own airline, there are clubs that offer membership to airline and industry staff - such as travelclub.com.au
As with class of travel, again, this varies from airline to airline. My understanding of Qantas' policy is that economy is available straight away, domestic business is available after a year (I think, but not sure), and International Business is after 7 years. Discounted First is not offered unless you are a Captain or senior management.