How reliable is the GAE Channel API /_ah/channel/disconnect channel_presence path?

Question

I'm using the GAE Channel API to connect to clients in a Javascript webapp.

I've found in testing that the /_ah/channel/disconnect handler can take as long as 5-6 minutes to fire after my browsers internet connection is dropped.

I haven't found any good documentation about how it works or what should be expected from it.

Does anybody have experience working with it? If so, is it guaranteed to fire? Does the latency increase with the number of total Channel connections open?

Any help is appreciated.

Here are the only docs I have that mention the channel_presence paths: https://cloud.google.com/appengine/docs/python/channel/ (Tracking client connections and disconnections)