Formal:
* Il y a longtemps que… (It's been a long time since…) - This is the most common and versatile option.
* Cela fait longtemps que… (It's been a long time since…) - Similar to the above, but a bit less common.
Informal:
* Ça fait longtemps ! (It's been a long time!) - This is a casual and friendly way to say it.
* On ne s'est pas vus depuis longtemps ! (We haven't seen each other in a long time!) - This is a good option if you're catching up with a friend.
* Je ne t'ai pas vu depuis longtemps ! (I haven't seen you in a long time!) - This is a slightly more formal option for addressing someone directly.
Other options:
* Il y a un temps fou que… (It's been ages since…) - This is more informal than "Il y a longtemps" and suggests a very long time.
* On se voit rarement maintenant. (We rarely see each other now.) - This suggests that you haven't seen someone in a while, and it is a bit more passive.
The best option will depend on the specific situation and your relationship with the person you are talking to.