@Nathanael Santschi As far as I know, you will be not able to retrieve next runtimes within Azure Update Manager. To retrieve the schedule associated with the maintenance configurations, you can use Get-AzMaintenanceConfiguration PowerShell command.
Get-AzMaintenanceConfiguration -ResourceGroupName smdtest -Name workervmscentralus
Location : centralus
Tags : {}
NamespaceProperty :
ExtensionProperties : {}
MaintenanceScope : Host
Id : /subscriptions/42c974dd-2c03-4f1b-96ad-b07f050aaa74/resourcegroups/smdtest/providers/Microsoft.Maintenance/maintenanceConfigurations/workervmscentralus
Name : workervmscentralus
Type : Microsoft.Maintenance/maintenanceConfigurations