Slaves were often manumitted upon their owner's death - assuming they had a good face-to-face relationship. If you working on a latifundium with no contact with your master, then it's unlikely you'd ever be freed.

Manumission was happening so frequently that Augustus had to pass laws to limit it. So if you were in their actual household and pleased the master, then quite often at their death (or even before that).

Walter Schiedel argued that over a five year period, about 10% of all slaves would be freed. Cicero (Phil 8.32) specifically says that a good, hard working prisoner can be expected to be freed within six years, Dio (53.25.4) says twenty. It is possible that the Late Republic of Cicero had such a glut of slaves that manumission was handed with with significantly more liberality, or, perhaps, both authors are making things up.

Beyond that, it is hard to say.