In case you need more than 2 lines, this should work:
k = 5
Buffer = ""
TextBox1.MultiLine = True
intSegmentCnt = Int(Len(TextBox1.Text) / k)
For idx = 0 To intSegmentCnt
Buffer = Buffer & Mid(TextBox1.Text, idx * k + 1, k) & vbCr
Next idx
TextBox1.Text = Buffer