I follow the RSS feed at the link below, which lists every addition to Netflix Instant Viewing.
The way I prefer to follow it is by email, using http://www.feedmyinbox.com/
. It can also be viewed in My Yahoo or your RSS viewer of choice.
I've been following it for almost a year with this method. Generally anywhere from 10-30 titles get added per week. It seems like more additions happen at the beginning of the month than the end of the month. Beyond that, there isn't any rhyme or reason to when things get added.
Also, FWIW, sometimes movies and shows will appear on the web site as much as a week before they appear in this feed. It isn't necessarily the fastest way to be informed, but it will insure that you don't miss out on something that you want to see when it gets added.