For a lifetime ago or since a lifetime?

Neither of those is correct. But I'm not sure what you're trying to say.

It's been a lifetime.

- It's been a lifetime since I bought Tab soda.
- When's the last time you went bowling? Seems like a lifetime.
- It's feels like a lifetime since I've gone rollerskating.

- The last time I visited a pumpkin patch was eons ago.
- It's been eons since I've gone square dancing.
- When's the last time I went to church? It's been eons.

http://www.dictionary.com/browse/eons