We require 7 days advance notice for greets. This is because we are not a professional organization with an office where a manager is organizing and where professional tour guides are waiting for customers. We are locals, volunteers, who love to meet people and show them their city.
So, if your request comes in it might (!) take a couple of days until our administrator sees that and distributes that among the volunteers. It will take some more time until it is clear who’s got free time to meet with you. And then the greeter communicates with you to work out the exact time and place when and where to meet.
Without that technical 7 days limit we had really many short term requests and it always hurt when we had to turn down such requests.