Informal:
* Ages: "I haven't seen her in ages!"
* Forever: "It feels like I haven't slept in forever."
* A while back: "I saw you a while back at the store."
* Dog years: "It feels like it's been dog years since I last went on vacation." (This implies a very long time)
* Eons: "It feels like eons since I last saw a movie."
* Since forever: "I haven't eaten pizza since forever."
* Donkey's years: "I haven't been to that restaurant in donkey's years."
More formal:
* A considerable amount of time: "It's been a considerable amount of time since I last spoke to him."
* Quite some time: "It's been quite some time since I last visited my family."
The best term to use will depend on the context and your intended tone.