I am not sure you understand how they work. You don't reserve them in advance, you request them when you want one. The APP then tells you how long it will be until the van shows up. Generally the app is reliable about that. However at times the app will not be able to match you with a car and you won't get a time estimate until it can (and that can be a while)
So no, you won't be standing around waiting for a car you reserved for a 9 PM pick up to show up, but at 9PM when you put in your request for a pick up you could find out it's going to be an hour before a car gets to you!