Hello,

Check these Articles:

1. About running on-demand scans http://www.symantec.com/docs/HOWTO27194
2. Configuring an on-demand scan for Windows clientshttp://www.symantec.com/docs/HOWTO27122
3. Setting up scheduled scans that run on Windows computershttp://www.symantec.com/docs/HOWTO55263
4. How to setup a custom scan that will scan only one single folder in Symantec Endpoint Protection (SEP)? http://www.symantec.com/docs/HOWTO59048
5. Configuring the scans of selected folders http://www.symantec.com/docs/HOWTO27159
6. About the types of scans and real-time protectionhttp://www.symantec.com/docs/HOWTO55226

Here are the Steps:

1.Open the Symantec Endpoint Protection Manager.

2. Select Policies from the far left.

3. Highlight Antivirus and Antispyware under the View Policies section in the middle column.

4. Highlight the active policy from the right side of the manager.

5. Click Edit the Policy under the Tasks menu is the middle column.

6. A new window will open.

7. Select Administrator-defined Scans from the far left menu in the new window.

8. Select the Schedule  tab. and chnage the time accordingly

Also, look at this Article:

Configuring SEP client options for end users to stop, pause, or 'snooze' administrator defined scans

http://www.symantec.com/docs/TECH93376