IF we are including RA2, then since about 1999Kaithofis said:approx. 2,5 years.
It all started with RA2 and AoE2 editors, after which I moved on to UT after I got a PC that could handle it.
There I learned the basics of the UEd, and just in time to begin the "real" mapping when UT2K4 was released.
After that my maps kept on sucking less and less