en un abrir y cerrar de ojos

Literally, “in an opening and closing of eyes”.

en un abrir y cerrar de ojos

1. (idiomatic) in the wink of an eye, in the blink of an eye, in the twinkling of an eye
   • 2020 October 11, Alejandro Ciriza, El País‎:

     No está fino, duda, se enreda. Y, en un abrir y cerrar de ojos, ha recibido una sacudida tremenda.

     [Djokovic] isn't fine-tuned, he's doubting, he's bamboozled. And then, in the blink of an eye, he's been shaken about like crazy.

2. (idiomatic) at the drop of a hat, at a moment's notice, in a heartbeat

• “en un abrir, o en un abrir y cerrar, o en un volver, de ojos”, in Diccionario de la lengua española, Vigésima tercera edición, Real Academia Española, 2014