I can answer that

Well, this article caught my eye and stirred my attentions:

Here’s How Long Foreplay Should Last, a Sex Therapist Says

My answer:  Continue to perform preliminaries prior to her participatory promise.

In other words, do it until she says she's ready.

Let's see what the sex therapist recommends:

"Everyone is different when it comes to sex and experiencing pleasure," Rayburn says. The Canadian study might illustrate her point: "Women and men have different sexual arousal and peak needs," she says, "so foreplay should not be put on a timeline, but rather be determined through preferences and response." In the study, the actual timing of foreplay varied from person to person—meaning a specific timeline doesn't matter when it comes to how long foreplay should last."

Yep, I pretty much nailed that one.

Hah.