Time constant. Transient thermal analysis

Hi, I don´t understand what the time constants in trasient thermal analysis mean. Does "mn" stand for the amount of "minutes" the motor takes to reach the steady state?

If so, there´s something I don´t understand. If you see the attached graph, time constant for the frame is around 12min which is around 700s. But if you look at the temperature vs time graph, temperature at that time is of around 216C and keeps increasing. This means the frame has not yet reached the steady state. Does anybody have any experience with this?

Thanks for your help. 

Jaim