am using strength of argument as an indicator of when to stop. since my thesis is theoretical, once i figured out the argument, half the work was done. now am in the process of writing it in a convincing manner / not a perfect manner. i used to be a perfectionist and in many ways i still am, but this PhD has helped me to learn that its ok for everything not to be perfect.
meanwhile - my personal deadlines also play a role in determining what is good enough, as i'd like to submit by the end of the year, preferably Sepf or before Christmas. i think there comes a point when you have to decide to finish.
one of my colleagues, now in his fourth year has not yet submitted and is still looking for the perfect thesis. to me, hardly anyone will ever read my thesis after examination, i can perfect it later in publications - whilst am teaching and being paid for it - and only concentrating on those bits that really interest me.