High Sierra Topix

I would guess it decomposed fairly rapidly - geologically speaking. It looks to be a glacial erratic left on that glacially polished granite. As a retired sedimentary geologist I'm not up-to-date on the glacial history of the High Sierra but, if I recall correctly, the last major glacial activity in the Sierra ended about 10,000 years ago. I suspect that that rock has been eroding ever since - so give it 10,000 years plus or minus a bit. A blink of an eye geologically speaking.

Well, I am also a "card carrying geologist" (professional licenses for Wyoming and California). However, my work career was mainly in groundwater in boring places like the Central Valley. My generic answer to the question would be it varies a lot due to the amount of surface area exposed to weathering and the "strength" of weathering due to location. Other factors like acid rain also play a role. But geologic processes are usually much slower than the average person deals with.