I always dial my espresso by taste, not time. Doing this will give you a lot more flexibility. That being said, I think the range where most of my coffees feel dialed is about 25-40 seconds.
I want to preface this by saying that I rarely change my shot from my standard 20g, so everything I’m going to say is conjecture, but I would guess that a single shot that takes 30 seconds to pull would be wildly overextracted. The reason why a double shot will take so much more time is because the puck of coffee that the water has to force its way through is much taller. Creating channels through a larger puck (or a puck made from much finer grinds) takes more energy from the water, so it should take longer. Once channels are made, the puck will quickly deteriorate, which is why the flow rate is so much faster at the end of a shot. That being said, you will definitely want to adjust your grind size for a single shot. Grinding finer than you would in a double shot will allow for a more even extraction in a single shot without overextracting the coffee, but it still shouldn’t take as long as your double shot.