Hello Julius, 

Thanks for reaching out to UVM Extension Master Gardener Helpline. 

The answer to your question depends on what the current pH is around your blueberry bushes. Adding too much of any acidifier can lower the pH beyond what is ideal – typically between 4.5 and 5.5 pH. You should plan to test your soil every few years if possible. More info on how to do that with UVM Extension here. 

If you have tested your soil and you're confident that additional ammonium sulfate won't have an adverse affect on your soil, the general recommendation is to add ammonium sulfate every year in spring before the buds begin to swell. The below resources have some recommendations on how much to fertilize depending on how old your bushes are. 

I hope that helps. Happy gardening!  

Resources: 

https://extension.illinois.edu/small-fruits/growing-and-caring-blueberries

http://www.metromastergardeners.org/files/tenminuteu/2014_Growing_Blueberries.pdf

https://blueberries.extension.org/blueberry-fertilization/