I added aluminum sulfate to my ES Hydrangea 2 weeks ago and it has not turned blue. How long does it usually take for a ES to turn blue after adding aluminum sulfate? Should I add more?
Better timing may have been to apply in earliest Spring just as your hydrangea was breaking dormancy - it isn't possible to change flower color within a couple of weeks; if you are in an area with alkaline soil and water it could take months (you might see results of your efforts the next growing season) or may not be possible at all.
Don't add more now...an excess of aluminum sulfate can be toxic to your shrub.
Here is a link that might be useful: Earlier discussion aluminum sulfate