My overly specific rough rule is about $20k in MRR. A deep dive on that here: I Hired My VP of Marketing at $20k MRR. It Wasn’t a Week Too Early. | SaaStr
I.e., usually much, much earlier than you might think.
Why? Assuming you are growing reasonably quick, a head of demand gen at even $20k in MRR can be accretive over the next 12 months. I.e., she can more than pay for herself.
In a recurring revenue model, you want to make every accretive hire you can. Don’t wait. Each $1 you bring in this year may be worth $10-$20 over the lifetime of a customer.
So if you can hire a VP of Demand Gen today and she can bring in say $200k in additional revenue from improving the funnel, BDR management, demand gen campaigns, webinars, events, whatever … hire her today!