Hi! According to http://wiki.mikrotik.com/wiki/Manual:IP/Firewall/Filter there are 3 params:
count - maximum average packet rate measured in packets per time interval
time - specifies the time interval in which the packet rate is measured (optional, 1s will be used if not specified)
burst - number of packets which are not counted by packet rate
So, limit 1,5 IMHO means 1 packet over 5 seconds.