We don't have such formula for that but we can do some programming to get that delay.
NOTE: We do not require the formula =IF(B2<>"",C3,"") for this programming, hence you should remove that formula.
1. Right click on the Sheet tab at the bottom on which you are working.
2. Select View Code ; a new window will appear.
3. Enter the Code as shown below:
Private Sub Worksheet_Change(ByVal Target As Range)
If Target.Address = "$B$2" Then
Range("B2").Select
If Selection.Value = "" Then
' Pause for 15 seconds.
Application.Wait Now + TimeSerial(0, 0, 15)
Range("B3").Value = ""
Else
' Pause for 15 seconds.
Application.Wait Now + TimeSerial(0, 0, 15)
Call MyDelayMacro
End If
End If
End Sub
Public Sub MyDelayMacro()
Range("B3").Value = Range("C3").Value
End Sub
4. In the Debug drop down menu select compile VBA Project.
5. And you are done , Now close the window and update the cell B2.
After every 15 Seconds the cell B3 is updated.
Thanks.