Thank you for the great recipe and the detailed instructions! I just have one question - how long should I whisk by hand until the mixture reaches...

The recipe switches to a rubber spatula for that stage. You really can't tell by time; too many variables, but she does give some guidance on what to look for; you might look for other citrus curd recipes (such as the lemon curd recipe she cites) for more guidance. I look for small bubbles to start bursting occasionally; they should leave a fairly persistent trail; the mixture should also be thickening noticeably, but will thicken more when it cools.