If you premix a palette and want to spread your painting sessions out of a couple of days - why not save the premixing work and use that paint again?
Put it in the freezer. Paint will stay fresh for a couple of days if frozen. You may wish to cover it with plastic wrap. Allow it to thaw for about 30 minutes and voila - fresh palette with all that pre-mixing work saved for day 2.
This palette was frozen for 3 days. The paint was fresh, even though I had frozen it for longer than I would normally have done. I noticed the umbers set up in about 5 hours after thawing, a little faster than normal.
Recently I even froze a whole painting - put it in the garage overnight so I could work the entire surface wet in wet - worked great!