You're right, Arnold is taking even longer
By a different "standard of measurement" I was actually refering to DnA Research.
Their "3Delight for Softimage" was originally released around the time of XSI 6.5,
after the what I think was the implementation of a new render API for XSI 6.0.
To put things in a different light, in those days (XSI 6.0) the following statement was issued in 3D World #95:
(...) XSI is now set to be made able to interoperate with Renderman, 3Delight, finalRender, V-Ray and Gelato. XSI 6.0 will be the minimum version required to use any of these plug-ins as they're released.
Well at least V-Ray offers unsupported backward-compatibility up to XSI 7
Time flies, when you're having fun...