A common question for anyone that is getting started with social media. Although the answer isn’t that simple because every platform is a little different. Not only are the platforms a little different but your audience may be a little different from one niche to another.
Social Caffeine came up with a nice infographic that you can use as a starting point. Monitor your metrics and tweak the times a bit.
Now that your social media is off to a great start learn more about using metrics to monitor the performance of your social activity. The Definitive Guide To Social Media Metrics