The shortest amount of time between contact and payment is 24 hrs. It stemmed from a client contacting me the day before the tax filing deadline. I was able to get the tax return done, submitted, and paid before the deadline. 

The longest has been close to a year. The client was a newly formed business without a direction of how they were going to manage bookkeeping and their business entity. After running the business for a few months, they reached out to me for my services as they found out how time consuming managing their financials could be.