Kevin, before I answer I need to have more information, such as vehicle type, operations type, and oil (as not all oils are the same) Without this extra information I would stay with a conservative number of 300 hrs

Extending oil drains on the engine is quite complex, and should only be done in conjunction with a used oil analysis programme. This is the only way to establish if the engines and oil combination will allow extended service intervals to be run in your conditions.
We have a lot of success of running upto double OE drains using out semi-synthetic fluid castrol Hypuron, however the type of fluid (mineral or synthetic) is not the only determining factor for drain intervals.