I would look at the directions on the package to see how frequently they recommend you reapply. I would also consider purchasing a soil pH Kit that gives the results in a numeric scale so you can better monitor the pH Levels. If the package does not give good directions, you can try applying it monthly (while the ground is not frozen) and write down the soil pH Kit's results down in a wall calendar so you can monitor your progress.

After some trial and error I had good luck keeping my endless summers blue. One bush I planted in a giant plastic pot that I buried (the bottom was cut out of the pot) this way I could change the the ph without affecting anything else.
The other I initially had the same problems you are having so I just mixed a lot of the alum sulfate in the ground around the root ball I must have put at least 1/2 cup if not more. This worked great and doesn't seem to bother the plants around it. From what I've heard it is fairly difficult to change the ph of soil so I figured I would use more than the amount on the pkg and its worked great for me.

Tricia, you may have been lucky. Using more than the recommended amount of a product is rarely a good idea...better to become fond of pink flowers when you prefer blue than to kill your shrubs.

Here is a link that might be useful: Previous thread, too much aluminum sulfate

it is funny how we all want blue flowers and pink is so pretty too:):)
I love blue too but I sort of let it do its own thing