Describes a state or action that has continued for a considerable period. Often used with verbs in the ~ている form or past tense.
I'm sorry to have kept you waiting for so long.
He lived abroad for a long time.
Sorry to have kept you waiting so long. I'll show you in now.
I'm sorry it's been so long since I last wrote; I hope you're well.
This system had been in use for a long time.
We thank you for your long-standing patronage.