EDIT: OK, let me help you arrive at the destination: your machine gets dusty and slow, just as if its electronics was failing. You take it to Apple and they fix it under warranty or ask for some $$ if it's out of warranty (given that they have to spend time doing it, time that you would be spending yourself, whether it's easy or hard to open up the machine). Calling it cleaning or repairing is splitting hairs. If it's degrading performance, it's a problem, if it's not, it's cosmetics.
BTW, all this fuss with cleaning fans — no one I know opens up their stuff and clean the fans until the device malfunctions. You are in the small small minority and no company, including Apple optimized for your habits.
The nastiest opponent to cleaning that Apple sells is the iMac lineup after the Late 2012 iMac. The iMacs before had their display glass attached to the frame with magnets, with 2012 Apple switched to adhesive... which makes servicing these really painful because it is easy to break that glass and pretty difficult to separate the front glass from the panel.
If you have cats, dogs or other beings that shed fur though around you, I'd recommend once every six months.
I don’t understand the insistence that we must regularly clean our computers, it seems daft. Maybe vacuum your house/office once in a while. The computer breathes the same air as you.