Treat Deployment Scheduling Software available time like a Collection maintenance window
The Deployment Scheduling "Software available time" is a one-time setting. Any newly imaged workstations built after that time will receive software update packages at any time during the business day. Unlike Collection maintenance windows, which only apply to the execution time for the software packages, there is no way (that I know of) currently to control the time of the download of the packages.
1) Provide a daily "Software available time" start time
2) Tie the software available time to a Collection's maintenance window
3) Define a maintenance window for "Software available for downloading".