The average California prisoner released in 2009 served about 2.9 years in custody, 51% longer than the average offender released in 1990.
"The cost to California taxpayers for this extra time served is $2.2 billion."
Adam Gelb is with the Pew Center on the States' Public Safety Performance Project which released the study.
He says time served for drug offenses and violent offenses grew at nearly the same pace from 1990 to 2009.
"For violent and career criminals, there's no question that the money is well spent. The issue is at the other end of the spectrum, that in California and in so many other states across the country lower level offenders have also been swept up in the net. This money can be better spent in other ways."
He says those "other ways" include: treatment programs; drug courts; and hiring additional probation and parole officers.
The state Department of Corrections and Rehabilitation says California is already moving in that direction with its "realignment" program; keeping lower-level offenders at the local level and out of state prison.