"no matter how long" is a perfectly valid phrase that is frequently used in written English. You could use it to emphasize that something remains true regardless of the amount of time that has passed. For example: "No matter how long it takes, I will never give up on this project.".
No matter how long you've been away.
"No matter how long it takes," he said.
"I will fight on no matter how long it takes.
I'll crack Proust's monster no matter how long it takes!
But no matter how long you've been around, it's still no guarantee of fame and recognition.
The beans are hard to stomach alone, no matter how long they're cooked.
No matter how long I sat there, I could never ever see his face.
Awesome tool! I started using it one year ago and I never had to look for another app
Ha Thuy Vy
MA of Applied Linguistic, Maquarie University, Australia