Max. possible flight time of V-Tol UAV

Hi, everyone

We are developing a project that is to make a new type
of UAV whose purpose is to observe and monitor land with the
following spec.

-Type : VTOL UAV, 
-Hybrid : Motor x 4 units, main engine x 1 ea   
-Wingspan : 5,000mm or more 
-Weight : 50~60kg or more 
-Required flying hour  : 6 hours or more 
-Cruise altitude 300 m 
-Cruising speed 80km/h
-Gas capacity : 25 liter
-Operation purpose : observing and monitoring from invader 
    or abnormal situation

And, we are wondring if Pixhawk can work for 6 hours and more
stably with the above condition.
And it will be great if anyone has done similar job and if can give
us comment.

Thank you in advance. We are looking forward.

Best Regards,
Joseph, Byun