The boot time schedule is for that time only when you reboot your comp, or when you turn it off and then next time you boot, it will run.
Boot scan is there for when Avast! itself asks for it. Not need to run it yourself.
If you want to schedule a fast or complete scan:
Open avast! > Scan Computer > Choose fast or full system scan > More details > Settings > Scheduling
BTW Welcome to Avast! Forums.