Larry,
I evaporated some samples, but may have left them longer than overnight, and possibly as long as a week. That was many years ago, when I was working long hours, and many weekends. The days just seemed to run together after a while.
I've also had good results by burning the samples with Castor content. Tried burning some mostly synthetic oil fuels, but the samples flared up, and splattered burning fuel outside the containers! I'm not sure what caused that, but thought at the time that the synthetic oil had ignited.
The muffin tin that I used, was just about ideal for the evaporation testing method, exposing a lot of surface to the atmosphere.
Bill