So can someone tell me ?

A few thousand years

I think her age have stop since she work for a system agency that oversees different worlds so maybe she's in an in-closed dimension where time is absent.

If i remember, the system said that our fl is eternally 18 in one of the chapters

Her physical body is 18 but she is a thousand years old
The system does not allow for her body to grow.
The moment you work for the system that's the time your body stops growing and slave away for life.
But you can also can ask for a vacation just like what our female lead did in the past chapter
She requested a 50 year vacation and she wouldn't grow old (again system does not allow age)
In the end her husband in that life died of old age perfectly healthy
And she stayed young, eh-

I'm just guessing but I remember her saying 3000 years old

What you said actually makes hella sense

Makes sense tho