Whats the cut off for starting app submission only with Xcode 7

Well, considering Xcode 5.1.1 is still allowed, it seems that 7.x will not be mandated for some time. Beyond 2016 I'd guess.

Why do you ask?

We are working on a large scale mobile application with many third party vendor SDK's integrated. I am trying to stay prepared for ATS and converting network layer from NSURLConnection to NSURLSession, ahead in time but same time we may have an intermediate release towards end of the year. If I know for sure that apple may put restriction on compiling with Xcode 7 for app submission I'd start working on that aspect right away or release upcoming build with Xcode 6.4 and later worry about optimizing it with Xcode 7.

This year in June Apple had put such restriction for compliance with 64 bit support.

That mandate is if you use current tools, not to only use current tools, so it's your choice.

I think you can take your time and follow up with something later this year if that fits your plan better.

Thanks for the guidance.