Agree with moondog. I don't know what the OP expects with "getting by", but unless someone is an extremely gifted individual, 7 hours will probably get you as far as very basic questions and answers. Not to mention the different accents will muddy the waters even more.

I wouldn't sweat it too much. Most people/places will immediately switch to English. The 'students' at Tiannamen Square will be very happy to try and practice English with you, or even humor you with speaking in Mandarin.