Houtsuite is a web + mobile app by which you can manage your Twitter, Facebook, Linkedin, Instagram, and more from one dashboard. Hootsuite lets you quickly schedule messages and easily manage your social accounts. Measure your social media campaigns, then improve ROI by doing what works.
I am using Houtsuite for a long time now and using schedule messages extensively. There is no limit how many you can schedule. I have schedule messages for months, which get automatically posted.
Its quite easy to use. Check out the free plan today by visiting https://hootsuite.com/plans/free