The freeway ends at Stockdale Highway near the western city limit of Bakersfield. Check out Google Maps
. Fancy traffic circle at Stockdale & 43. Stockdale now designated 58 to I-5, Rosedale relinquished.
If you look at the satellite view, it's clear that right of way has been acquired alongside the canal south of Stockdale curving to connect with the freeway at Stockdale with land for a interchange at Stockdale. It extends most of the way to 43. Not clear what happens west of there or when. But if you dig through the (possibly online) Caltrans and Kern COG archives you'll probably find an environmental document that gives you a good idea. With the truck traffic that will use the new freeway, I'd guess that at least some extension will happen within 10 years.
Funny tidbit: back in the early CAHSR days when it was proposed to shoot straight down I-5 bypassing the Valley cities, Fresno and Bakersfield were supposed to build rail transit lines out to stations on the HSR - at their own cost, no help from HSR. In Bakersfield's case at least, that was supposed to be a light rail line running in the median of the new 58 freeway.