Hi there, depends if colic is an issue. If it is I would burp every ounce or so. Otherwise, maybe burp halfway through the bottle and then at the end. I think firm back rubbing is the best burping technique, back slapping can make reflux worse if it is an issue.
Also, most babes will let you know if they are uncomfortable throughout a feed, by wriggling, and generally seeming uncomfortable.
HTH a little