How long does it take for a View to pick up a new tag?

At most, 1 minute.

The Historian notifies each view when a dataset changes. This includes the addition or removal of tags, or properties. Each view determines if they are interested in the dataset that changed and ,if so, will rebuild automatically.  Once a view rebuilds then it too notifies all other views in case they were built upon it and need to rebuild. This all happens in the background, but the longest you should have to wait for a view to rebuild after tags are added to a dataset is 1 minute, depending when it occurs in the timer cycle. Once a view rebuilds, it will not rebuild again for 5 minutes. If you single click a view in the views tile, you will see a property in the bottom left called Last Update which is the last time the view rebuilt.