When you’re writing it? There’s no difference between writing it and saying it. It’s not spelled with a c if you say it and with a q if you write it out.
I think “quando” might be slang. You also see people using “q” for “que”. It’s probably like in English how people say “wut” for “what” or something.
In Italian, it’s “quando.”
I took four years of Spanish. Dictionaries, websites, signs, textbooks, closed captions, and pretty much everywhere where people can spell correctly say it’s “cuando.” 