Apparently, I've gone "too long" with my 10-speed Ultegra chain, at about 1/16" over 12 inches. Though my mechanic offered to change my chain, suggested that for the price of the 10-speed chain (retail $40), I might as well keep going for a bit, since the cassette (retail $80) too would need changing, and as worn as it is I wouldn't be happy with just a new chain.
This is my first 10-speed bicycle. Said mechanic mentions that the Shimano Ultegra chain has always been roughly half the price of the Shimano-series cassette. This seems to have been the case back in the 9 and 8-speed Ultegra days.
I've gotten about 2500 miles on the chain, so assuming I do wait about 500-1000 more miles, that's 3500 for $120 versus, say changing the chain every 1600 miles and one cassette and another chain at say 3500 for $160.