I think it's too early to tell. Maemo 4[1] got 5 updates (not counting beta and minor updates, sources: http://en.wikipedia.org/wiki/Maemo#Release_history, http://tablets-dev.nokia.com/nokia_N810.php, http://tablets-dev.nokia.com/nokia_N800.php) over a 13-month period[2].

We are now in the 3rd to 5th month (depending on how you count) since Maemo 5 reached users and have had one major update (and a couple of minor ones) with a second major update imminent. Some bugzilla comments (https://bugs.maemo.org/show_bug.cgi?id=6394#c5) already indicate that there is also going to be at least a "PR1.3" update, but nothing is known beyond that.

[1] I think counting Chinook and Diablo together for this is appropriate, YMMV.
[2] Which in itself is fine. My major gripe is that Nokia never came out and said that 43-7 was the final update, if they had we could have started working on community updates over a year ago.