Agree with that, but would also like to throw in a scenario where there wasn't a sterile environment, but conditions were harsh enough for all present life to d

Agree with that, but would also like to throw in a scenario where there wasn't a sterile environment, but conditions were harsh enough for all present life to die, leaving you with a 'sterile' thin soup containing the building blocks of DNA based life. That might shave a bunch of time off of the time needed.