AutoFill, så længe der er tal/tekst i række 1

Option Explicit
Dim rC1 As Range, rC2 As Range
Sub test()
Set rC1 = Cells(Selection.Row, 2)
Set rC2 = Cells(Selection.Row, Range("A1").End(xlToRight).Column)
Cells(Selection.Row, 2).AutoFill Destination:=Range(rC1.Address & ":" & rC2.Address)
End Sub

Option Explicit

Dim iCount As Integer
Dim rC1 As Range, rC2 As Range
Sub test()
    ' fra række to til sidste række i A
    For iCount = 2 To Range("A" & Rows.Count).End(xlUp).Row
        Set rC1 = Cells(iCount, 2)
        Set rC2 = Cells(iCount, Range("A1").End(xlToRight).Column)
        rC1.AutoFill Destination:=Range(rC1.Address & ":" & rC2.Address)
    Next
End Sub