When do you present a website UX survey?

I'm wanting to build a usability survey on a website that's used by mortgage brokers.

It's a website where users will come not to browse, but to find mortgage product information while a client is sat there with them; so it's usually a short visit. The survey cannot be targeted as it's an unauthenticated website.

How soon after page load is the best time to pop up an invitation for a short survey?