I think inflection point would be determined by:
a. Funding - I mean, you need to pay these guys a bunch. Equity can get you only so far
b. Stage - I think hiring a stellar VP of Sales with a half-made product is not going to play out well.
Also, does the startup have a semblance of a repeatable business model (which tends to be correlated with later rounds of funding)?
If you know by putting in X sales reps with $Y sales productivity per rep you can achieve $Z (X*$Y) in sales then that is a good case for hiring a VP of sales and have him build a sales organization under him.
any other ideas?