Upvoting and downvoting should be 100% symmetrical.
Any kind of restrictions (time allowed, number, frequency) placed on upvoting should also apply to downvoting.
Any kind of restrictions (time allowed, number, frequency) placed on downvoting should also apply to upvoting.
Giving one type of vote any "advantage" over the other creates a systemic imbalance.
Perfect symmetry between the voting types should be preserved at all costs.