What time does PSN store usually update?

EST or PST time zones.

For me in the UK it's usually Wednesday afternoon.

Majority of the time it updates between 5PM-9PM EST time zone on each Tuesday of the week.

Is there anything good coming in the update?

Usually there will be a preview on each Monday what we can expect. PlayStation blog is pretty standard website to keep track off.

Here is this weeks update, as of this writing, yesterday: http://blog.us.playstation.com/2012/11/27/playstation-store-update-267/