correct sentences

Hello!
I would like to know if these 2 sentences are correct: “How long have you been here?
I have been here since twenty minutes to 8 (o’clock.)”
Thank you for your time!

Yes, they are correct, though ‘minutes’ would often be left out. In addition, the ‘o’ clock’ on the end of your second sentence would not be voiced. It is assumed. It would only be voiced if the time were an exact hour.
I have been here since twenty to eight.
I have been here since eight o’ clock.