When and how can pilots waive wake turbulence holds in the US?
The AIM 7-3-9 lists the required separations. Behind a heavy aircraft, the departure hold is 3 minutes and it may not be waived. Behind a small aircraft, the hold may be waived on pilot request only:


  3. Additionally, appropriate time or distance intervals are provided to departing aircraft:
  
  (a) Two minutes or the appropriate 4 or 5 mile radar separation when
  takeoff behind a heavy/B757 jet will be:
  
  (1) From the same threshold.
  
  (2) On a crossing runway and projected flight paths will cross.
  
  (3) From the threshold of a parallel runway when staggered ahead of
  that of the adjacent runway by less than 500 feet and when the runways
  are separated by less than 2,500 feet.
  
  NOTE- Controllers may not reduce or waive these intervals.
  
  b. A 3-minute interval will be provided when a small aircraft will
  takeoff:
  
  1. From an intersection on the same runway (same or opposite direction) behind a departing large aircraft,
  
  2. In the opposite direction on the same runway behind a large aircraft takeoff or low/missed approach.
  
  NOTE- This 3-minute interval may be waived upon specific pilot
  request.
  
  c. A 3-minute interval will be provided for all aircraft taking off
  when the operations are as described in subparagraph b1 and 2 above,
  the preceding aircraft is a heavy/B757 jet, and the operations are on
  either the same runway or parallel runways separated by less than
  2,500 feet. Controllers may not reduce or waive this interval.


EDIT: The AIM 4-3-10(f) (intersection takeoffs) also addresses this question, including suggested pilot phraseology for waiving the hold (emphasis mine):


  Controllers are required to separate small aircraft (12,500 pounds or
  less, maximum certificated takeoff weight) departing (same or opposite
  direction) from an intersection behind a large nonheavy aircraft on
  the same runway, by ensuring that at least a 3-minute interval exists
  between the time the preceding large aircraft has taken off and the
  succeeding small aircraft begins takeoff roll. To inform the pilot of
  the required 3-minute hold, the controller will state, “Hold for wake
  turbulence.” If after considering wake turbulence hazards, the pilot
  feels that a lesser time interval is appropriate, the pilot may
  request a waiver to the 3-minute interval. To initiate such a request,
  simply say “Request waiver to 3-minute interval,” or a similar
  statement. Controllers may then issue a takeoff clearance if other
  traffic permits, since the pilot has accepted the responsibility for
  wake turbulence separation.


Wake turbulence in general is considered a key safety issue by the FAA and there is a large amount of information in the AIM, the ATC guide and elsewhere.