How Long Does It Take A First Degree Burn To Heal?

i am a 16 year old girl.my skin got burnt while waxing my upper lip day before yesterday morning. i noticed the burn the same night and its blackening everyday. my skin doctor said it was a first degree burn and advised me to use 'fluster' twice daily. if i do so how long will it take for me to get rid of the burn? please help.i am hoping it will take a weeks time cuz i hav a wedding to attend.thank you