The update time is irregular and i hate it. It can get frustrating and demoralizing thinking that you have not gain more backlinks that is counted by them.
google never shows all backlinks anyway on purpose so people can't use the data for seo purposes. that's why you only see all your backlinks in Yahoo!, etc.
It depends. Sometimes it takes only one week to update links. I got a backlink from a high pr site just 10 days ago. Voila! It showed up in webmaster tools today.