All commercial networks have ad breaks of a pre-set length. OneHD in Australia is streaming the races live through their website. The ad breaks there (usually just a blank screen with a message about the race returning soon) co-incide with the ad breaks on the television - and they're all of an identical length.
Well, that leads to an interesting development that occurred at ITV. Fans were so outraged at the ad-breaks that ITV agreed to do something they'd never done before; cut out of the middle of a break (even an advert) on restarts and other important in-race events. The affected advertiser(s) would then get shuffled along the ad-cycle.
It worked but was too little too late for most fans, I expect many of them cheered when it was announced shortly after that Aunty had got F1 back!