I got lucky a couple of Fridays ago. Called Sinderellas, Lauren picked up and told me that she was immediately available for the next hour, so I jumped into my car, drove like a screaming bat outta hell and got there in time. So sometimes you may get lucky.
That said, I think for any of the popular SPs....Lauren, Lilith, Rosanna, Sydney, Melanie, Shawna, Gabrielle, etc...drop-in is likely not going to happen with them, so I'd book at least a couple of hours in advance...the earlier in the day or evening, the better.