...Personally, I'd run that out to 30, but I'm just a diver...
It really takes only about 1 - 2 seconds. In my training, I was switching pretty instantaneously from the primary to backup in probably one second, and I was told to slow it down.
I was donating the primary with my right hand and by the time the "victim" had control of the reg, I had already located my backup with my left hand and popped it in my mouth. Easy peasy and I'm sure under a second.
They asked me to wait until the victim had taken a breath or two, so maybe a few seconds.
One of my instructors could switch to his backup without hands by just ducking his chin. I've never tried.
Keep in mind that this was right after Alec reiterated to me his concerns about 2 divers without a reg in their mouth at the same time, since I was switching to a long hose, and I discussed it with the instructor.
...Well, it's not really a race, and it's not about the time it normally takes, what you need is time when things don't go as planned and you are there with the reg out of your mouth trying to sort out what to do next...
You can take a moment to donate when you're ready. It doesn't have to be the split second that you get the signal. If you just exhaled, you can inhale and then donate. You can also have your backup ready in your hand before you take your primary reg out.
I find that no different than with practicing the old buddy breathing. You took a couple of breaths and then passed the reg back. You likely chose to pass it back after you inhaled rather than exhaled.
It's also just like a reg re and re. Many of us have had a reg sculled/kicked out and it was a non-issue, even if you just exhaled.