3 years. You have 3 years from the date of the accident to make a claim for compensation.
Extensions are available in some cases, but are extremely difficult to obtain.
If you are under 18 years at the time of the crash, different timeframes apply. The limitation date is 3 years after your 18th birthday if you have not already commenced a claim. And the only way to commence a claim before your 18th birthday is if a parent of guardian commences the claim on your behalf.