The Schedule attribute is a BLOB, so setting that from PowerShell is probably going to be a bit of pain. I suspect that you can set one of your site links manually with the GUI and then use the value stored in "Schedule" to replace the "Schedule" value on other site links.
I haven't tested this code, but I suspect this will work to reset the replication interval to the default (15 minutes) and the replication schedule (provided you get the appropriate value to specify in the "Schedule" attribute).
Get-ADObject -Filter 'objectClass -eq "siteLink"' -SearchBase (Get-ADRootDSE).ConfigurationNamingContext | Set-ADObject -Replace @{ReplInterval=15;Schedule=your_schedule_value}
The Connection object in each server in each site has a "Schedule" attribute, too. I'd recommend clearing these attributes (returning them to 24 x 7 schedules, effectively) and setting your replication schedules on the "siteLink" objects. This will allow you add another server to the site w/o requiring you to set the schedule on that individual server. The siteLink object's schedule will "trump" the schedule specified on an individual server's "Connection" object.
You could clear these schedules back to default by running:
Get-ADObject -Filter 'objectClass -eq "nTDSConnection"' -SearchBase (Get-ADRootDSE).ConfigurationNamingContext | Set-ADObject -Clear Schedule