How to have vba execute every 10 minutes?

Consider:

Public RunWhen As Double
Public Const cRunWhat = "my_Procedure"

Sub StartTimer()
RunWhen = Now + TimeSerial(0, 10, 0)
Application.OnTime earliesttime:=RunWhen, procedure:=cRunWhat, _
     schedule:=True
End Sub

Sub StopTimer()
   On Error Resume Next
   Application.OnTime earliesttime:=RunWhen, _
       procedure:=cRunWhat, schedule:=False
End Sub

Sub my_Procedure()
    MsgBox "hello world"
    Call StartTimer
End Sub

all in a standard module..............be sure to run StopTimer before exiting Excel

NOTE

The "minute" argument in TimeSerial is the second argument.

You should use this pattern:

Sub my_Procedure()
    MsgBox "hello world"        
    Call test ' for starting timer again
End Sub

Sub test()
    Application.OnTime Now + TimeValue("00:10:00"), "my_Procedure"
End Sub