If x = 745/999, then
999x = 745
1000x - x = 745
This equation tells us that the fractional part of 1000x and x cancel upon subtracting. This means the block of repeating digits must be 3 digits long.
To verify this, suppose we write x = 0.yyy… , where y consists of 3 digits. Then
1000x = y.yyy…
1000x - x = y.yyy… - 0.yyy…
999x = y
x = y/999
which means y = 745.
