"extend the duration of" is correct and usable in written English.
You can use it when you are referring to making something like a time-limited activity or a deadline last longer. For example, "I am trying to extend the duration of my membership by six months".
Soon, Congress is likely to debate whether to extend the duration of unemployment benefits.
Last week, Hexion sued Credit Suisse and Deutsche Bank in a bid to force them to finance the deal and extend the duration of the commitment agreement.
The engineering company ABB Ltd. plans to sell convertible bonds worth 850 million Swiss francs ($601 million) to partly refinance bank debt due at year's end, as well as extend the duration of its debt and diversify its financing.
Real-time monitoring will likely "widen the net" (and extend the duration) of control to include misdemeanor drug users and other minor offenders who otherwise would be left to informal surveillance or none at all.
In November it decided to extend the duration of unemployment benefits paid to older workers, which will encourage employers to dismiss more during the next downswing.
Generous loan covenants give managers more flexibility to work through their debt, and lenders are keen to extend the duration of loans rather than call them in and face more write-downs.
"I thought I knew what the sentence meant," said Dave Beinlich, describing a section that said that "preparation" to drill was enough to allow Chief Oil and Gas to extend the duration of his lease.
The effect of that patience and choosiness is to extend the duration of unemployment for all workers, which slowly makes even those with the "right" skills less hirable.So there are two things to say about this.
The overpressurized balloon is quite useful to extend the duration of the flight.
Heat transfer through the burner was suppressed to extend the duration of the experiment.
HR managers would not want to extend the duration of an interview if it didn't serve any purpose.
