If you have a large number of users for whom you need to extract presence, you have to split up your request because the presence identifiers are part of the URI and the URI can only be so long. It doesn't say how long the URI may be though..so is there any other way than trial and error to figure out how I have to split up my requests?
I've inquired with the engineering team for any known supported max length (as well as max # of contacts per request), but I suspect trial-and-error is your best quick-and-dirty approach. AFAIK max URL length of 255 is usually assumed to be the safe minimum, but as there are unlikely to be proxies or other weirdness between your app and CUCM, that number of probably quite a bit bigger.
Also worth considering is the SOAP version of the API, since the contact list is POSTed as an XML chunk, you won't be bound by the URL length limit - though you are then of course subject to the (currently unpublished/unknown?) limits on max XML body size and/or max # of contact element limits...