Title says it all, really.
I'm asking because I've got a subscription to MaxMind, and I'd like to know how important it is to stay on top of updates to their geolocation database.
They get reassigned very, very rarely, if at all. But have you considered that MaxMind's data may actually be improving over time? I don't think that they currently have enough data to assign a region to each given IP address and if they could, there'd still be some room for improvement by making the regions smaller and smaller.
Thus, I think, it all depends on what you want from the data and what you do with the information once you have it. Are you sattisfied to know the country or even continent? Or do you need to know the exact city?
It depends on how what regions you are interested in. IP addresses are first assigned from IANA to RIRs (there is one RIR per continent). Those RIRs can then distribute the IP ranges to LIRs (companies or organizations giving out IP ranges to "end customers").
IP ranges can be given back from the LIRs to the RIRs. But the RIRs (almost never) will give the ranges back to IANA since they split up their /8 range to several LIRs.
So you can say that the IP range will stay on one continent. And once they are assigned they rarely move from one customer to another (and with this from one country to another).
But what happens more is that new IP ranges are assigned to new customers. And then you would have to know in what country it is used now. This is probably the main reason to get a MaxMind subscription.