Yes there's an adjustment. Go here: http://drop.io/cx500_gl500
. Get the factory manual. The procedure is the same for all CX500s built 1978-1981. Read. Follow the procedure exactly.
How often? I do it every year or every other oil change, whichever comes first. I adjust the valves at the same time. The whole operation takes about 1/2 hour and must be done with a stone-cold engine.
If you adjust the tension regularly, you will probably find a need to go into the engine for some other reason first. When you do, tensioner blades and a new chain are part of the 'triple bypass' ritual that CX riders generally perform whenever one of the three weak links (water pump, stator, timing chain) starts causing problems.
Chains that are adjusted less often wear out much faster. It's hard to overdo this one. There is no penalty other than your time for adjusting the chain 'too' frequently as long as you do it right.