Is it bad to have left my tampon in for 12 to 14 hours because i had a light period and no cramps?

Likely not: While tampons left in for long periods of time can increase your risk of toxic shock syndrome (fever, chills, severe infection) I have never seen a case in my career. If your period is that light, you might want to consider using a mini-pad? Likely many women have done what you describe with no problem, but you may want to consider changing your method. Hope that helps!