No one really knows, they update it randomly.
4 times a year is the pattern till now. An update is expected by the end of July since the last was at the end of April and the one before that at the end of Jan.
Next pagerank is expected around Jul 21
Hey ! Thanks a lot. Can you tell me how did you come to know about the date ??
You can predict when something will happen by looking at its pattern. The website has enough information on when Google has updated its PageRank database before so it can predict (with surprising accuracy) when it will happen again.