Depends on ambient temperature, ISO, and what raw processor you use. Also depends on your personal feeling as to what is usable.
Normal ambient temperatures, base ISO, and Capture One 7: I'd say 60 seconds is usable. Some might be okay with 70 seconds while others will only be happy with 40 seconds (i.e. multiple people can look at the same set of files and make different assessments as to which ones are usable).
Why don't you call me on Monday and arrange for me to send you a set of raw files shot at various exposure lengths, and then I can screen share with you and show you, in Capture One 7, how I would handle these files to make them each look their best - and then you can decide for yourself what's usable and what's not.
Or even come in to the office and shoot some files yourself at different lengths of time so that you know everything about the files from conception to final evaluation.