Release dates are not always published publicly and the availability of that information online depends entirely on which state and facility she is in.
Many state departments of corrections do maintain public inmate search tools on their websites that display projected release dates. If you know which state she is incarcerated in, searching for that state's department of corrections inmate locator is the fastest starting point. Most of these tools are free and searchable by name. The BOP inmate locator at bop.gov covers federal inmates specifically and is one of the more comprehensive public databases available.
InmateAid also maintains a database of about 1.5 million inmates pulled from public correctional records. Searching her name there may surface her current facility and release date if that information is publicly available for her jurisdiction.
If the online search does not produce results, calling the facility directly is the most reliable path. Ask to speak with a counselor or the unit team secretary and request the release date information. Be polite and patient when you call. Staff are more responsive to calm and respectful inquiries than to frustrated or demanding ones. Identifying your relationship to her and explaining that you have not been in contact helps frame the request in a way that is more likely to get a helpful response.
If you do not know which facility she is in, start with whatever county or city she was last known to be in and contact the local jail or the state DOC to trace her location from there.