85% of sentenced time is standard. A portion of the sentenced time (at the end)may be spent at a halfway house. Regrettably 85% of 31 months is 26.35 months which is obviously more than 1 1/2 (18 months) years.
Of course, every answer is based on the question asked and requires a more complete context. This answer should not be relied upon to make a legal decision. Seek the advice of an experienced criminal defense attorney before acting. Law Offices of Raymond G. Wigell, Ltd. Defenders of the Constitution since 1975/ Aggressive Creative Defense Strategies/ Website: www.waaltd.com 24/7 --(708) 481-4800.
Call your Niece's attorney and have them explain the minimum that she will have to serve and when it is likely that she will be able to enter a half way house.
It's possible that she will only serve 18 months because she is getting credit for time served in custody before she was sentenced. Her attorney can tell you that.
Answers posted here are suggestions only, and are not intended to replace advice given by an attorney that you hire.