Well, silence itself is powerful all the time, its of mouth only or of mind. It's one of the reasons behind the popular quote “silence is Golden”. If you are curious about the right time to use silence for your benefits, it's when your senses give up. In other words, if you can silence your mind, your senses will be under control. But if you can't, definitely stay silent when things are not under your control. It's the time when your silence can be more powerful and give you the best answer.