Dodge Avenger 2008 P0456 code. Changed the gas cap. The malfunction light remained on for 5 days. The light is now off. Should I assume that was the problem?

If nothing was done to clear the diagnostic trouble code, then it is possible that you have resolved the concern. However it may take some time before the evaporated emission system performs its testing again as a routine operation while driving and operating normally. Only then will the system be able to say that there's no problem. I would recommend continuing to drive the vehicle is normal, and if the light comes back on warning you for check engine, you will need to readdress.