30-50ish composers, focused only on their solo piano compositions, and the recordings thereof, along with the releases on which those recordings appear.
I've been considering this question for several weeks, and have come to realize that without doing more in-depth user research, I'll remain uncertain what will be acceptable to our user base.
I'm a bit surprised at the range you present - 2 months versus "just" a day. Would you be willing to elaborate a bit on why you chose those durations, and the relative complexity of meeting a requirement at either end of that scale?
To be honest, I'd think that in 2016, we'd be talking about 1 day as the floor, and then calculating why near real time is probably too expensive for the immediate future, but something to start working towards in small increments.
hypothetical usage scenario
Suppose, for a moment, that the api enhancement I suggested as one possible solution is deemed worth pursuing by the community. I'd expect it to be utterly non-impactful to make a single request for edits related to a collection on, say, an hourly basis. Maybe the response to such a request indicates 3 changes, but only 2 are relevant to the particular data attributes I've cached - leading to 2 follow-up requests to get that refreshed data, assuming the original response didn't already contain sufficient information for me to know what to update in my local cache.
And keep in mind that these 1-3 hypothetical hourly requests are all in place of each of my users hitting the api directly, possibly numerous times per minute. If other client developers were to start adopting a similar architecture (i.e. local caches of data), we could reduce the load on the api by a few orders of magnitude.