You can schedule your client PCs for updates from your SEPM
if you are using liveupdate server for updation (internal or external).You can
do this configuration in your liveupdate policy.

There are multiple ways in which client get updates...

Client checks the server based on heartbeat... So you can set the heartbeat interval based on ur need. There is no schedule option available for this.

You can use internal liveupdate server to update clients.. You have a option to schedule it. Its available under liveupdate policy....

Work around for schedule updates....(Give a try.. it should work)
Set any  non existing ip as a GUP. Now when a client check in with the server its get the GUP info and tried to contact it. Sicne you have configured non existing GUP it wont download the updates.

Now you have to install and configure a liveupdate server and assign it in liveupdate policy. You can schedule the update as per your need...

Now clients will get update as per the schedule..

REF:
https://www-secure.symantec.com/connect/articles/i...

The default configuration is that the clients will take the updates from SEPM every 4 hours.
i.e. SEPM will download updates every 4 hours.
As the SEPM downloads the definition it distributes it to the clients.So in order to schedule it further you will have to schedule the time when the SEPM will download the updates.

@Vikram
We can schedule the update of the server once in a day... But there is no control over the client update.. Whenever all the pc's are switched on it contact the server and gets the updates....

Customer always complains that pc's are taking updates. They want to Schedule these updates in the afternoon (after 1 O clock).... 

Update schedule for client should be added in the product...
Like liveupate schedule there should be a schedule for update from SEPM also...

One of the BIGGEST things missing in both SAV and SEP - seems silly still not here!!

Customers always asking us why, what they perceive so simple is not a setting e.g. clients at location A update at 12:00.

Regarding this problem I had posted one idea. So that in future releases it may get included .You can also vote for that. Web URL: https://www-secure.symantec.com/connect/idea/sched...

Thanks aravind.... I have voted... 

I feel this would be a great help to add this functionality. I tried to vote but got page not found.  Please make this happen ASAP!! Thanks.