However I still don’t know the point of the time unit in the rate variable.
This is probably to simplify rate calculations for long time periods.
The rule with ‘dst-limit’ is true and action is triggered when packet rate is below rate set by you.
In opposition to ‘connection-limit’ when action i triggered when packet rate is above limit.
Someone mentioned in the forums that the time units are actually 1/10th seconds.
AFAIK this was never explained by MT, so maybe it’s time to do this 
Here is a link to ‘dst-limit’ example by Chupaka:
http://forum.mikrotik.com/t/ddos-story-or-warning-use-conection-limit-with-caution/49743/1
HTH,