The exact amount you are entitled to reclaim depends on various factors, including the distance traveled, the type of truck (number of axles and permissible total weight) and the relevant EURO-standard.
According to current knowledge, the general amount of refund is approx. 4% for claims for the last three years, i.e. from 2019. However, depending on how the competent courts will rule on this, the amount of refund may sum up to 7% and claims may also be brought for the period beginning 2005. However, the question of the statute of limitations has not yet been decided.