Based on the level of respect or formality, you can say "How far is it?" in Korean in 3 ways:

• Informal: eolma-na meoleo? (얼마나 멀어?)
• Formal: eolma-na meonayo? (얼마나 멀어요?)
• Very formal: eolma-na meomnikka? (얼마나 멉니까?)