be in (something) for the long haul

To be dedicated to doing something for a long period of time. This phrase usually describes one's commitment to someone or something. Jill knows that obtaining a PhD is going to be tough, but she said she's in it for the long haul. Once we sign this agreement, we'll be beyond the point of no return. We're committed for the long haul after that. I love you, and I'm not going anywhere. I'm in this for the long haul.

be in something for the long ˈhaul

be willing to continue doing a task until it is finished: Trust me when I say we’re in it for the long haul.