Pagerank is continually updated and is, in the main, what affects your ranking position - though there are other factors.
The visible part is the Pagerank Toolbar update that Google plays with and the last update was around three months ago as I recall. The longest time between pagerank updates has been 267 days so potentially it could be another 6 months before a PR toolbar update.
It's a shame in a way that Google works this way because they could update it continually. It does keep people looking and waiting though which is good for Google.