In many ways, it depends on exactly what integration you're using and what that integration supports. While there is some support for this in Dialogflow CX, it probably isn't as automatic as you want.
If you're building using one of the telephony integrations, there are some agent settings that let you set the "No speech timeout" period. At the end of this period (5 seconds by default), if the caller hasn't said anything, then a no-input event will be triggered and you can use this to repeat the message, prompt if they're still there, or take other actions that may be contextually appropriate.
If you are using a text agent, there isn't any such timeout built in. However, if you can keep track of this on the client side (ie - in whatever is sending the messages to Dialogflow), then you can determine if there has been no input for the predetermined amount of time and send an event to Dialogflow that will trigger the reprompt.
For example, if you're using Dialogflow Messenger, you could setup a JavaScript event that sets a timeout when it gets a response from Dialogflow and clears it when the user sends a request. If it ever times out, it could bring up a message re-prompting the user.