My oral was about 2.5 hours- It was very indepth on wx (all types of wx charts/ wx phenomena, icing/types), follow by the x-country plan which incorporated (charts, scenarios, loss comms, alternates) Then more REGS, Nav Systems and Components (GPS/VOR/NDB/ILS) then when over the a/c systems I flew. I did it on a multi so I also talked about critical engine, principles of flight(inop engine) etc, and that was about it... At the end I was exhausted, but at the same time relieve I was over with it. And the flight was a piece of cake. The only hard part was that I had to do a Partial Panel Single Engine Approach follow by a Circle to Land all with partial thrust on one engine.
*If you can try to observe a few oral checks- I found them helpful. Is pretty much of to the examiner on how much he think is good enough, Ive observed a few oral that were 1 hour top. And others that were long and more indepth like mine.