45 minutes for Int'l to Int'l in FRA according to DL reservations. I was there a few days ago and went between LH and DL terminals in about 15 minutes.
As for standby, only try it if you have hand luggage and there are no guarantees. I have been told it is possible but never experienced it. With checked bags on a different DL flight, you are assuredly going to be on the flight that your bags are booked on.