Depends on average demand. I'd say 60 minutes is a good rule of thumb. If the route can only support 2x daily, try 120 minutes. Per a previous post, if the tolerance for flights is 35 minutes and you schedule 3 flights at 500, 530, and 600--the system will view it as one flight since 500 and 600 are within 35 minutes of 530 versus the 500 and 600 flights being viewed as two flights with the 530 merged into one or the other. 60 minutes is a safe number, 120 minutes if only 2x daily to be safe.