There was approximately 420 years between their births. Abraham was born in about 1946 BC and died in about 1771 BC. Moses was born in about 1526 BC and died in about 1406 BC.
Consequently, there was about 245 years between the death of Abraham and the birth of Moses.
See “The Times: a chronology of the Bible”, chapter 3, for an explanation of how these dates are calculated.