Because the spline wrap can't repeat its offset, the unsolvable problem @bezo mentioned applies, so perhaps this way might not be suitable.
You didn't show us your setup, so I don't know how suitable this is, but if you were doing this by cloning onto splines you could use Xpresso to calculate a Cloner Rate multiplier based on the spline length in relation to the longest spline in the scene. This could, in theory at least, get all your arrows moving endlessly at the same speed and without any stopping half way along them.
But I am as bad at maths as I am at Xpresso, so we need someone like @jed to just quickly tell you how to do that :)
CBR