How much tan is too much tan you ask, well the girl in this photo (taken from pintrest) was doing the the 100 layer challenge and boy that must feel horrible.
Everybody has a different skin type and tone and some people can take a lot of tan and not look funny but some people will look like a road cone if they put too much tan on. If you are applying tan at home and your skin starts to look like your cracking then your putting too much tan on.
Body builders and dancers will often have lots of tan applied, this looks great under those bright lights but no so great under normal conditions. The trick to getting it right for your skin is either your tan technician should know or trial and error with at home tanning.
Remember a tan is supposed to enhance your looks not change you entirely.