The Nebraska DMV sets the proof period for your case and states it on your notice. The clock runs only while a valid certificate is on file, so a gap does not shorten the period, it extends it.
More detail
We deliberately do not print a number here, because the period depends on the action taken against your record and the only authoritative statement of it is your own DMV notice. What is worth understanding is how the clock behaves. It starts when a valid certificate is actually on file with the state, not when the violation happened, not when the court case closed, and not when you bought a policy from a company that turned out not to make filings. If the policy cancels mid-period, the insurer reports it and the clock stops until a new certificate is on file. Drivers who let a policy lapse twice frequently find the obligation running a year longer than they expected. Continuous coverage is the only thing that makes the period end on schedule.