Formal:
* Je n'ai pas eu de vos nouvelles depuis un certain temps. (This is the most literal translation and is appropriate for formal situations.)
* Il y a longtemps que je n'ai pas eu de vos nouvelles. (This means "It's been a long time since I've heard from you.")
Informal:
* Je n'ai pas de tes nouvelles depuis longtemps. (This is more casual and can be used with friends or family.)
* Ça fait longtemps qu'on s'est pas parlé. (This means "It's been a long time since we talked.")
Other options:
* J'espère que tu vas bien. (I hope you're doing well.)
* Comment vas-tu ? (How are you?)
You can also add a specific detail about the last time you heard from them:
* Je n'ai pas eu de nouvelles de toi depuis [date/event]. (I haven't heard from you since [date/event].)
The best way to choose the right phrase will depend on the context and your relationship with the person you're talking to.