I would think that most nodes get the majority of their votes in the first day to a maybe a week after they are posted when they are on the Newest Nodes or the Front Page. After they roll off those the chance of getting a vote is a lot less.
If you base it off the $NORM for that day or week (possibly weighted more towards the day it was posted) you should account the bulk of the voting to be done on that node.
If a node continues to get votes after this time it makes sense to rank it higher (or lower for down votes), since that would indicate (to me at least) a strong staying power or usefulness of that node.