I've gone through the announcement and release dates of all NVIDIA products since the TNT, and they are on approximately an 11 to 12 months cycle between new cores (NV15/NV20, NV20/NV25, etc.) with a refresh stuck somewhere in between, varying from 4 to 8 months (NV16, NV20-Ti, NV28 etc.). The only real exception was from the GeForce to the GTS, which only took 8 or so months instead of 11.
So, I'm not sure what you are claiming. 18 months is too long between new cores (considering NV15 and NV25 to be different enough to be included as new cores) and it's too short between new DX architectures (NV10, NV20, NV30).