Paint dries rapidly on the canvas and the palette as well. Retarding agents and gels can help in slowing the drying on both. Acrylics appear to dry darker than when they are first applied, but this is not actually the case. The milky white look of mediums and gels, when wet, disappears when dry. This gives the illusion of the paint darkening as it dries.
Acrylics are about the most durable materials available.
The question about expected life has been tested repeatedly in laboratory-controlled exposures to heat and light. Acrylics do not exhibit the same changes as oils, leading experts to guess that the life of acrylics is far greater than the materials that were used for centuries before their discovery. In summary, if you are looking for a rapid drying, opaque or semi-opaque medium, with which you can create any color or texture, that is extremely durable, cleans up with soap and water, will adhere to virtually any porous surface, and can be augmented with many foreign materials for personal texture applications, you must try acrylic paints. They offer more than you could imagine. If the separation is for more than 20 minutes, seal the palette in an airtight container, with the dampened paper towel over the paint.