There are 2 bosses in Rooftop Run.  In order to reach the first, you will need to complete 2 rooftop and 1 street section. In order to reach the second, you will need to complete 1 more rooftop and 1 more street section.  All of this will need to be done in one run. Transitioning from rooftops to streets happens automatically. You can also use the ninja smoke bomb to jump forward through each of these sections to reach the bosses faster.