For ownership change limitations (ownership change as defined by the IRS is an increase over 50% of stock ownership) triggers Section 382 of the Internal Revenue Code.
382 allows the new owner to utilize pre-ownership change NOLs but limited at "long term tax exempt rate." The percentage is determined by the highest value of the month the ownership change occurred and the preceding 2 months.
The limitations don't reduce the total amount of NOLs.
$6 billion in NOLs total subject to 2.65% (Jan 2016 rate) annual limit = $159,000,000 annual use. At that rate (using simple math) it would take 37.73 years to utilize the entire value of the NOLs.
Keep in mind - NOLs carry forwards expire in 20 years.
At the moment - WMIH Corp (according to their 10K) doesn't believe the NOL has any limitation.