I think you can derive this from your desired growth rate and the structure of the market. Many markets are top heavy: a few giants dominate and then there are many many tiny customers and often not much in between.
In one market, let’s say there are 1,000 customers who are small companies themselves and who will each pay $10,000 per year to use your service. And then there are 100 companies who are big and will spend $100,000 per year. The little companies are simple sales and buy using their credit card. The big companies are a complex sale and require a top down approach with complex approvals.
You can win the first 1,000 customers organically, but because not everyone is ready to buy all at once, let’s say it takes 5 years to win them all - in other words you’re adding 200 x $10,000 = $2M of new ARR each year. By the way, this would be a fantastic market all by itself.
But sadly, with each passing year, your organic growth from small customers brings a lower percentage growth rate. $0 - $2M is exciting and so is $2M - $4M. But $4M - $6M is only 50%. And $6M - $8M is only 33%...
If you want to keep up 100% annual growth, you’re going to need to bring in some big customers starting in year three so that you can go from $4M to $8M instead of just $4M to $6M. To win that extra $2M in ARR, you’ll need 20 large customers along with the 200 small customers from organic growth.
Now, I don’t know what your market looks like, but I’d say that the time to bring in top down sales is around when you’ve found product market fit as evidenced by steady organic growth. If you are getting repeat sales from small customers who are really happy with your product, then, chances are, big customers would love to have what you make too. They just have to be sold to.
In other words, I would not wait. Start doing top down sales as soon as your product is hitting its stride with repeated happy customers.