The TAF would be valid for well after the TTF finishes. You can't "get a TTF", it is an observation that will remain in effect for up to three hours after it is made. the TAF will continue to be in effect well after the TTF expires. Any location with the facilities to enable a TTF (is is continually watched) will more than likely be a Cat A recipient (min 24 hr validity TAF) and as they are reissued every six hours....you get the drift.
Once the TTF expires, well then so does the ability to not consider the 30min buffers.
Another consideration is that if you are arriving within 30 or 60 minutes of the INTER or TEMPO (respectively) that creates an operational requirement (ie. alternate) finishing (including the buffer period), then you only need enough holding fuel to get you to the end of that buffer period. (BIG INHALE!)
For example;
If your TAF's TEMPO finishes at 0300Z and you are arriving at 0315, then you only need 15 mins holding, ie. until 0330 when the buffer finishes.
