It mainly depends on the students´ attitude. In my case, it took me one term to radically change their attitude (too much Spanish spoken in class, students got in late,...), but now it´s them who make me speak English about 95%  of the time and even tell each other to speak English!

I think you should make the class and the topic interesting for them so that they are motivated to response and use more English,never forget to praise them for using English in the classroom.

I tell my students if they use more English in the classroom they get more points and more points they get the chances are there for I give them a present at the end of the term or course to those with higher scores.