The formula for Dinitrogen tetroxide is N₂O₄ while that of Nitrogen Dioxide is NO₂. The balance reaction is
N₂O₄ ⇄ 2 NO₂
The reaction is actually an equilibrium reaction, but the forward reaction is the spontaneous one. In this decompostion reaction, the reactant is N₂O₄. At the start of the reaction, there is an abundant amount of reactant and zero moles of product. As time goes by, the reactant is slowly converted to NO₂. So, you would expect that at the start the forward reaction has a very fast rate. As time goes by, the amount of reactant will deplete so the rate slowly decreases until it levels to zero. If you graph it in a qualitative curve, it would look like that in the picture.
The answer is: It starts high and gradually decreases until it reaches a rate of zero.