How much does insurance brokers make on trailers from premiums and for how long ?

What type of "trailer" are you talking about a travel trailer, a mobile home or a utility trailer.

In general on items like car and house insurance ( in US) the broker makes from 10 to 15 percent of the premium paid as commission, They get this on every payment.
Some brokers hire agents, who only get that amount on the first payment, then the broker makes all the renewal.

So if you can be more specific it will help