the erlang is the cumulated time of traffic carried in the channels of the observed system.
1 erlang = 60 minutes
2 erlang = 120 minutes
3 erlangs = 180 minutes
4 erlangs = 240 minutes
40 erlangs = 2,400 minutes
400 erlangs= 24,000 minutes
and so on… there is no formula.
you measure 3 erlangs between 9am and 10am, it meands that there were 180 minutes of usage of your system at that hour.