Pretty much whenever...  We get it, your time is precious (so is ours).  That's why we plan around your schedule to minimize business disruptions.

You make money from selling your products and/or services, not marketing.  But... you're not making sales without marketing.

Or, if you want to get Seussical about it

Time away from what makes you money, 
is time you aren't making money. 
However, you aren't making money, 
doing what you do to make money,
unless you make time to tell the people,
about what you do that makes you money,
so that you can have time doing the thing that makes you money,
for the folks you'll do it for that will pay you money.

Let's

We get it, your time is precious (so is ours). 

You make money from doing what you do, not marketing what you do...

time spent marketing is time not spent on sales, but there's so sales without effective marketing so you're stuck in an endless loop.  Unless...

You've got a partner invested in your success as much as you are.