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Unfortunately you can't set the specific time frame for SEP clients to receive definition updates from SEPM or GUP.
SEPM will pass on the definition updates depending upon Push mode or Pull mode & Heartbeat interval settings.
But if you are already configuring GUP then there shouldn't be any concern especially about bandwidth usage.
The GUP will download definitions on-demand for itself and any clients configured to update through it. The GUP will cache all downloaded content according to the settings in its LiveUpdate policy. Clients that have been configured to use a GUP will download definitions directly from the GUP instead of SEPM. By this method, bandwidth is conserved. There must be sufficient bandwidth between the GUP and the SEPM to allow the GUP to download the full and delta definition packages being requested by SEP clients. The larger the spread of definition revisions used by the clients, the larger the bandwidth utilization between the SEPM and the GUP.
Though bandwidth usage can be significantly reduced by using GUPs strategically, it is still important to ensure that GUPs are positioned in the network to maximize their effectiveness. GUPs should only be configured to provide updates to for clients on their local network segment. The GUP must have sufficient bandwidth to deliver content packages of up to 260 MB to the clients it serves up to 3 times a day.
In any case if you still want defintions to pass on only once in a week then can check for other options like party tools.
Enables third-party tools such as Microsoft SMS to provide updates to client computers securely.
To use this feature, you must set up the Symantec Endpoint Protection Manager to use as a staging server for content. This staging server does not require that the clients be connected to it. Configure the server to download updates on a periodic schedule. If you use continuous, the server downloads the latest updates when they are posted.
By default, the updates appear in the Default group's clients' content outbox folders. These folders are organized by content type. You can then pick up one or more content packages from the content outbox folder and deliver it to the client's inbox folder.
To ensure that only third-party management tools update client computers, disable the other LiveUpdate server options on this page.
Note: Third-party content management settings are applied to Windows clients only.