Depends on what you mean by cutting. If you mean cutting them back because they have got too big, pruning advice is given here by the RHS.
https://www.rhs.org.uk/advice/profile?PID=516

Depends on the variety of Hydrangea - if its H. macrophylla, you don't cut those, really, you just clip off dead shoots and flowerheads round about April, when growth has begun. But if its H. paniculata, you can cut that right down after flowering if you want and it will still flower next year.