You should ideally be scheduling 20% of your content. which should be your sales and broadcast messages.
But when should you post these messages out to your network?
Use http://www.tweriod.com (a free tool) to see when your network is online the most.
For example, I know that my followers are online the most at 11am and 6pm – so my sales posts will be scheduled for these times each day for optimised times.
It is also worth adding a note in your diary to check this on a regular basis. Only 3 months ago, the best time for me to schedule a post was 7am and 10pm. So things change as your network changes.