When I kept my PC on or near the floor, I cleaned it out every 4-6 weeks. Now that I have moved my PC up to the desk it is far less frequent. As mentioned above, I do deep cleaning about once a year or so as well.
My typical cleaning is to power the PC down. Remove any dust filters if equipped (I mostly do not use these) and take them to the sink for a good washing. I take the side covers off the case and carry the PC outside where I blow it out with a cheap electric leaf blower. I typically will take a microfiber rag and clean the surfaces of anything left over, spray and clean the glass (if equipped) and done.
The once or so a year cleaning involves the above as well as taking things down like the fans and physically wiping and cleaning them, brush the motherboard and such to get any stubborn dust and cobwebs. If I have a system so long, about every 2-3 years I repaste the cooler/block unless I experience some issue before then. Of note, I generally do not mess with the graphics card paste or cooling pads unless there is an express issue with it.
Be aware that if you use a compressor to clean with, that it have an oil/water filter inline.
Each system and environment can be different. I do recommend using some manner of temperature monitoring such that you can see when dust is causing an issue.
edit- If you happen to smoke or vape, increase the frequency of what one might consider the yearly cleaning. In particular, vaping near your tower will cause a very gummy dust-oil goo to form on your filters, fans, heat sinks and just generally everything they blow on.