Good rule of thumb is 0.4C or lower.
You can go much higher than that and manufacturers are figuring out how with variable C rate charging. Tesla is safely up to 4C. The reality is you can safely charge as high a rate as you wish as long as you stay within the parameters of the manufacturers temperature rating.
.4C - 1C charging is a thing of the past with thermal controlled BMS systems that charge batteries according to their temperature rating. The charging process of lithium ions passing from the cathode through the electrolyte to the anode is catching up to the oxidation "discharge" rate. Technology such as thermal resistant sold ceramic electrolytes are going to be becoming commonplace in the near future.
Quick charging with no detrimental effects is becoming more commonplace as charging/battery technology is keeping up with each other. I'm amazed at not only the solar generators but stand alone battery and BMS advancements that are allowing charge rates that were unheard of just few short years ago.
I predict in just a few years we will be seeing 10C+ charge rates on 500+amp hour battery banks and under one hour full charge from full discharge.