Based on the level of respect or formality, you can say "How long will you be here?" in Korean in 3 ways:

• Informal: neo-neun yeogi-eseo eolmana itseul geoya? (너는 여기에서 얼마나 있을 거야?), neo-neun yeogi-eseo eolmana itseul geoni? (너는 여기서 얼마나 있을 거니?)
• Formal: dansin-eun yeogi-eseo eolmana itseul geoyeyo? (당신은 여기에서 얼마나 있을 거예요?), dansin-eun yeogi-eseo eolmana itseul geopnigga? (당신은 여기서 얼마나 있을 겁니까?)
• Very Formal: dansin-ggeseo-neun yeogi-eseo eolmana itseushil geopnigga? (당신께서는 여기에서 얼마나 있으실 검니까?)

- how long: eolmana (얼마나)
- you (subjective): ne-ga (네가), ni-ga (니가), neo-neun (너는), dangsin-eun (당신은), dansin-ggeseo-neun (당신께서는)
- to be: itda (있다)
- here: yeogi-eseo (여기에서), yeogi (여기), yeogi-e (여기에)

Please refer to How to pronounce Korean Alphabet for the English pronunciation in this answer.