If you want to change the time range for the tabs in PRTG you can go into the Server Administrator tool and set them there under Memory Usage. Please note that there are preset values that can be used and once you have changed these, you will need to recalculate the graph cache since this will have changed.
The options that you see under the Memory Usage tab are all that are available. If you want to get a report with a graph for a 7 day period, or any amount of time for that matter, you can use the Historic Data tab or reporting to get those results. This can also be done through the API with an API call similar to:
Disclaimer: The information in the Paessler Knowledge Base comes without warranty of any kind. Use at your own risk. Before applying any instructions please exercise proper system administrator housekeeping. You must make sure that a proper backup of all your data is available.