How late can I do an away

No. It's not too late to be considered by that program. You can do an away up until interview season (late november/dec), if your only goal is to improve your chances for that particular program. On the other hand, if you want to get letters from that away rotation that will make it into your ERAS application, Sept is probably the last month to do an away (just for the logistics of getting the letter sent to your school, scanned in, etc.)

FYI, I did my first away last year in Sept (got a letter from this month for ERAS) and did my second away late oct/early nov (no letter obtained).

I completely agree with Jet-X. You might want to take into account that at certain programs (HSS, HJD, MGH, UPENN, etc... I know, east coast bias) lots of students will be doing aways from july-october, so you might get less face time with certain faculty/residents. But I don't think that you should let that prevent you from doing aways at places you might want to spend 5-years. I did two aways in July and November (got letter from first); matched at neither, loved one iffy about the other, so I think maximizing the number of aways to find a program that fits you is a priority. Every program puts on a show on interview day (or at least they should) so only way to really find out about a program is to see their daily routine.

Check out the post about advice to medical students on ortho rotations, it sums that up well.