Source: Pew Charitable Trusts, June 2014
From the summary:
More than 1 in 5 state inmates maxed out their prison terms and were released to their communities without any supervision in 2012, undermining efforts to reduce reoffending rates and improve public safety. In the past few years, at least eight states—Kansas, Kentucky, New Hampshire, North Carolina, Ohio, Oklahoma, South Carolina, and West Virginia—adopted reforms to ensure that authorities can supervise all or most offenders after release from prison.
These policies typically carve out the supervision period from the prison sentence rather than add time for it after release. This allows states to reduce prison spending and reinvest some of the savings in stronger recidivism-reduction programs.