Repetition is a total no no when it comes to Search Engine Optimisation, but for social media, it is an effective way to raise and retain awareness of your brand and encourage engagement. No-one is online 24/7 and due to low organic reach on most social media platforms, repeating your announcements will ensure that the majority of your followers will see your latest news.
The answer to how often and when can be found by looking at the analytics in your chosen platform. Some are more detailed than others, (e.g. Facebook will give you a time when the majority of your followers are online, whereas Twitter will give you demographic information once you have enough followers).