Hi Ayaz,
Running backups will always eat up some freem memory, impact the LAN if backing up servers using Remote Agent for Windows Servers, and chew up some CPU cycles.
How long this will take depends on:
1. Size of your backups
2. Speed of your backup device
3. Backing up the local server only, or via a LAN
Best thing to do is run the backup, and just keep an eye on any impact on user performance. From my side, if I miss a backup, I will run a backup during the day just to make sure I have a backup for the night before. This also assists in clearing Exchange logs, failing which Exchange would go offline.
Hope this helps?