If asked the question, which cannot be answered with couple words, and you refuse to answer, you could reply with "Долго писать".
But if you decide to reply (for example, within some instant messaging app, or you are in the SMS chat), you reply differently:
"Погоди-ка...", "Сейчас...", "Минуточку..."
More often we use "Долго объяснять", which also means the refusal to give an explanation.