I see no reason to wait even three years.
Of course, we don't even have the death penalty in the Netherlands. Which is, I think, a mistake. It shouldn't be used lightly, and the main issue to guard against is the danger of executing someone who later turns out to be innocent -- but in this case, we know with absolute certainty that Volkert van der Graaf is the murderer. He is unrepentant. They should have just erected a gallows and dealt with him.
Same for Mohammed Bouyeri, who murdered Theo van Gogh. Same for those guys who killed crime reporter Peter R. de Vries last year. (And since one of them is a cousin of a henchman of Ridouan Taghi, I think it fitting to just execute Taghi as well. There can be no sane person who questions that he and his criminal organisation were behind this.)
...Of course, would it help? It would certainly deal with those specific thugs, but others are already lined up to fill the resulting vacancies. Civility and sanity have already lost. We live now in an age of radicalism, thuggery, terrorism, barbarity, mass shootings and mental illness. Perhaps, speaking of the Netherlands in specific, if a man like Fortuyn had gotten his way, he culd have turned things around. At least to a sufficient degree, to make further -- more gradual -- solutions feasible in the long term.
But that was twenty years ago. And he got killed for his efforts. In the following decades, things have only gotten worse and worse. And even now, there is no political majority for such a simple thing as "bring back the death penalty for the worst scum out there".