If the scheduled duration of an event is changed after a bet is placed but before the start of the event, then all bets will be void. Some markets refer to the length of time until an occurrence in the event (e.g. time of first goal).
Betfair has a standard commission rate of 5%. That applies to almost every sport, in every country, around the globe & except Australia. Due to the differing fees charged by the NRL and the various racing commissions, Betfairs commission rate in Australia is variable.