It's hard to give a direct answer to your question. But I will try to give a realistic answer.
As stated in the "Easy access rules for large aeroplane CS25" under EASA regulation:
Except as provided in sub-paragraphs (d) and (e) of this paragraph compliance with the
provisions of sub-paragraph (a) of this paragraph must be demonstrated in flight over the
acceleration range –
(1) –1g to 2·5 g; or
(2) 0 g to 2·0 g, and extrapolating by an acceptable method to – 1 g and 2·5 g
That's the normal range of load factors for a modern airline; of course an aircraft can be design to withstand more positive or negative G's, but most of them have an upper limit of +2.5g and -1.0g (in clean configuration), as stated in the regulation.
The following fundamental requirements have been developed around the experiences of metallic airframes and remain the basis for the certification of composite airframe structures. These require that the structure (by test and/or analysis) demonstrate the following capabilities:
• Static strength:
– Design limit load (DLL), no failure or unacceptable deformations. DLL is normally the maximum load anticipated to be placed on the structure in its lifetime. – Design ultimate load (DUL), no failure, although limited permanent deformation is acceptable; DUL = DLL × 1.5 (generally).
The limit of +2.5g and -1.0g will be our DDL, so if we "remove" the safety factor (1,5, but can be higher) the Max G load will be +3,75g and -1.5g. if you fly at this load factor the aircraft will not broke apart or will have catastrophic failures, but deformation can occur.
Let's talk about turbulence:
Extreme turbulence in reality is not part of the categorization of levels of turbulence (At least in EASA rules), the upper limit is the severe turbulence:
Turbulence that causes large, abrupt changes in altitude and/or attitude. Aircraft may be momentarily out of control. IAS fluctuates more than 25KT. (>1.0g at the aircraft’s centre of gravity)
So assuming a fluctuation of 1g (positive or negative) inside turbulence, we can assume that we will be able to reach +2.0g or -0g passing trough this turbulence.
If the turbulence will be "extreme" (let's assume a fluctuation of 2gs), we will reach a max positive G load of 3g and a negative G load of -1g.
As you can see, we still below DUL limit. I can tell you that probably the aircraft will not have any deformation in that range also if above DDL, this is because we are talking of instantaneous accelerations and not continuous and constant load factors (DUL and DLL are based on that).
The aircraft will however be checked after the encounter of severe turbulence.
Reassuming, is very hard that turbulence will cause any significant or severe damage to a modern aircraft. (Severe or Extreme)
But to give an answer to your question I can give you an example:
China Airlines flight 006 experienced vertical load factors of up to 5.1 g on 19 February 1985. The aircraft landed safely and there were no fatalities, but there was some damage:
From the NTSB incident report:
The wings were bent or set permanently 2 to 3 inches upward at the wingtips; however, the set was within the manufacturer's allowable tolerances. The left aileron's upper surface panel was broken and the trailing edge wedge was cracked in several places.
5.1Gs was probably the top of the load, but during the recover they were above DLL and DUL for a while.
This however didn't cause a catastrophic failure of the wing section.
I don't have any document or personal information or experience about the instantaneous or the continuous g Load to cause the snap of the wings, but I think that we are talking of +7/8g -4 continuous and +10-11g -5-6g instantaneous.
I hope that I answered your question.
