I bought several of these for my art classroom.  These are affordable paints that are better quality than Crayola watercolor trays or Prang for school use.  The kids love using them as well.  I use them for grades 3 - 8.  I generally let two people share one wheel stack.  I let them dry out between uses before I put the lid on.  They've lasted several months already and I'm hoping that they will last the whole school year.  Watercolor is one of my favorite mediums and I tend to linger on that longer than any other...so we'll see if they last.  I'll try to update to let you know if they did indeed last the whole school year.  We use them quite heavily.  :)