The first answer is that you should post to GMB every time there is an update that you feel like customers should know about. If you have a new promotion starting up, that would make for an excellent GMB post. Other important business updates like a change in hours or modification of COVID-19 precautions you are taking would also make for great GMB posts.
The second answer has to do with making posts for SEO reasons. The truth is that the solution here is unclear. Based on the experiences of Local Viking customers, we suggest that you post once a week and to leave each post up for seven days. You can use Local Viking to automate this process for you and you can type many future posts up in advance.