How much protein should I feed my laying chickens?

Too much protein is consequential to the health of your chickens. That can result in kidney failure. Protein should be fed in accordance to breed, age and health needs. Too little can lead to improper growth, muscle issues, reduced egg laying. 

Here is the protein levels to stay with-in for standard chickens. You can verify this with reputable feed companies such as this site. They usually state this information on the websites. They have a lot of scientific research to back up their feed formulas. 

• Up to 6 weeks: 20-24%. 18% works but 20-24% helps improve growth success. You can add supplemental protein if needed. Remember if you feed anything other than chick feed to chicks it's important to add grit so they can digest it. 
• 7 weeks to point of lay: 16%-18% protein. I stick to feeding chick feed 18% until 4 months because they're still growing and susceptible to growth issues such as lame legs and hips.
• Once they have started laying: 16% - 18% protein. My go to is 16% and I adjust according to their needs throughout the seasons. My primary reason is because I toss them treats with protein here in there. So this way I am able to stay in a safe protein range. 
• Mature hens: 16%
• All-purpose, over 6 weeks old: 16% protein - You should supplement protein to meet guidelines above if using this feed. 
• Winter: 20-22%
• Molting: 20-22%

Meat and Game birds require a higher protein feed than standard layer birds.

You shouldn't try to balance out the protein annually such as thinking if they need 16% in summer and 22% in winter, then 20% annually would work. 

It doesn't work that way since the protein is used up daily or stored daily. You are essentially not providing enough when they need it most and overdoing it; then them storing it and not flushing it enough when they need it least. So it's important to adjust as they need it throughout the year.

It would likely be fine to feed 18% annually, just nothing above 18% and limit treats with protein. Free range may do better on 16% due to access of a lot of protein free ranging. 

Remember location matters too.. for example in some regions summertime doesn't get sweltering hot and in others it does.. so always make your best informed decision based on all aspects to consider..including location.