Google Voice voicemail will answer after 25 seconds -- there is no way to adjust that timing.
If your answering machine counts rings (as opposed to waiting the equivalent time), you can change the ring cadence in the OBi, so the fourth ring occurs and the machine picks up just before voicemail. You might try changing Ring Settings -> Ring Profile A -> Ring Pattern 1 -> RingPattern from 60;(2+4) to 60;(2+3), which knocks a second off each ring. If that works, you could then try e.g. 60;(2+3.5) -- if it works, you'll have an extra couple of seconds to answer.