I'm not sure whether you mean:
The actual speed through Z-space itself, unrelated to normal space.
The relative speed from point A to B in normal space. E.G. you spent a year travelling 100 LY in normal space distance, so your relative speed was 100 LY per year.
As we spent timeless time traveling through the blank white non-distance of Zero-space...
The Andalite Chronicles, p. 70. Emphasis mine.
I'm not sure if it's even possible to measure speed in Z-space. What would you measure against? There is absolutely nothing there. It's just a great nothingness. Although in #18 The Decision, an Andalite ship (in an
Ellimist scheme extremely unlikely coincidence) discovers and resuces the Animorphs while they are stuck in Z-space. So it does work a little bit like normal space where you can observe and interact with objects. This leads me to assume that spaceships can travel through Z-space with about the same speed they can travel through normal space with. There is no air or resistance in either, so I don't see why the speed would be different. How fast is this? On page 60 of The Andalite Chronicles, Elfangor, while in the vicinity of Saturn, explains that it would take "many days" to travel to Earth through normal space.
The distance from Saturn to Earth is (very) roughly 1.5 billion kilometers. (This also, very much depends on the orbits. It can vary between 1.2 and 1.7.) We can make a wild guess say that "many" days means something like ten days. That means that they would have an average speed of 150 million kilometers per day. This, of course, is an extremely rough guesstimate, but it can give you some idea of what ballpark we're talking about.
So, assuming that Andalite spacecraft (or those of a similar level of technology) can travel about as fast in Z-space, we can assume maybe somewhere between 100 million km per day and a few hundred million km per day.
Z-space is constantly "reconfiguring" itself. There are many references to this throughout the series, for example:
A Zero-space rift had opened up between planet Earth and the busy
centers of the galaxy. That happens sometimes. It meant that Earth,
rather than being days away, was now months and months away.
The Andalite Chronicles, p. 323.
So, relative to normal space, there is no fixed speed while traveling through Z-space. Even the Andalites can't predict how these rifts are going to occur. So a destination which is very far away one day, can be very close the next, and vice versa. This may mean that theoretically, there is no maximum speed. Maybe a reconfiguration one day could leave far corners of the galaxy a minute away from each other through a Z-space tunnel, resulting in a very high speed relative to normal space. While there may be a limit to what such reconfigurations can cause, I don't believe it has been described in canon.
In The Andalite Chronicles, Prince Alloran's ship travels from Earth's solar system to the Taxxon home world in what appears to be a very short time. He decides they are going there, announces it, has a pretty short conversation with his passengers, and suddenly they are there. No further waiting is described. This suggests that given a "lucky" configuration of Z-space, it's possible to travel incredibly fast. It would depend on the configuration or Z-space at the moment of travel. With the right configuration, it may be possible to travel across a galaxy in one minute. While that is a bit speculative, I'm not aware of any evidence that there's an actual limit to how fast these configurations can cause travel to be.