ducted heating cleaning

Hi guys,

Need your advice. My IP settled in nov 2009 and the tenants moved in jan 2010. The previous owner had the ducted heating running when I inspected the property. Now the tenant is requesting that the ducted heating to be cleaned due to dusts in the outlets. I thought this is regular maintenance that the tenants need to do themselves by vacuuming the outlets. Based on your experience, who is responsible to clean the outlets on the regular basis? Is it the tenant or landlord? Is there any regulation/law that requires the landlord to professionally clean the outlets?
If you dont know the state of the ducts, maybe its best to get it serviced and cleaned.

I'm of the general understanding that cleaning the vents and chutes/boxes (in floor heating), is up to the tenants (as long as they know the vents pop in and out). But as the landlord you should be the ducts cleaned out as well at regular intervals.
please excuse my ignorance (our PPOR has a split system but my lucky tenant has ducted), what would you say is a 'regular' interval to service a ducted system (motor service + duct cleaning), we've had our split for 10+ years and apart from cleaning filters, we havent touched it