“After forty years had passed, an angel appeared to Moses in the flames of a burning bush in the desert near Mount Sinai.” – Acts 7:30
“And in the sixth year of the third week of the forty-ninth jubilee thou didst depart and dwell in the land of Midian, five weeks and one year. And thou didst return into Egypt in the second week in the second year in the fiftieth jubilee. And thou thyself knowest what He spake unto thee on Mount Sinai.” – Book of Jubilees 48:1-2
The situation here is much like that in Acts 7:23 previously. The Scriptural account (Exodus 3:1-10) does not provide us with an amount of time that the Prophet Moses spent in Midian; St. Stephen uses Jewish oral tradition, as reflected in Jubilees, to fill in the blanks. Interestingly, both St. Stephen and Jubilees, probably both following the same Jewish rabbinic tradition, have divided the life of the Prophet Moses into three 40-year periods: the first in Egypt as Pharaoh’s daughter’s son; the second in Midian more or less as a fugitive; and the third as a Prophet leading his people out of captivity, through the desert, and into the promised land. This harmonizes perfectly with Scripture’s confirmation that Moses was 120 years old when he died (Deuteronomy 34:7).