You should use "had been". This is because "Has been" is the present perfect tense. "Had been" is the past perfect tense. The present perfect is used for something that began in the past and continues through to the present time. Past perfect began in the past and was completed in the past, so because the context of your arrival in the cinema is in the past, that is what you should use :-)

.....had been on for 10 minutes. Your reference point is in the past: "When I got to the cinema". So you must use "had been".