No estoy seguro de cuán a menudo debería estar tomando esta medicina.

[morbrorper]

English Translation

I’m not sure how often I should be taking this medicine.

Wiktionary says this about cuan: “Rare in modern speech, usually replaced with que, tan, cuanto, or como de”.

I wonder if it’s more used in the Americas than in Spain.

[cdemora]

I would probably use “qué tan frecuente” here, but I’ve heard “cuán” as well (Latin American Spanish).