The answer is somewhat of a wild goose chase, but it can be found in the Manual for Court Reporters/Recorders, Section 2: Chapter 7: Records Maintenance, which sends you to the Michigan Trial Court Case File Management Standards, which sends you to
MCL 600.2137(3)], which in short form says:
For circuit court matters, it's 15 years for all felony cases; 10 years for all others. Since officials don't usually separate their civil steno notes from the criminal notes, saving them for 15 years is the safest bet. If a transcript is produced, then the steno notes only have to be kept for a year.