How lethal is meningitis?

Depends. A bacterial meningitis can cause death in a very short period of time, 24 to 48 hrs if untreated. If you or someone you know has any concern for this whatsoever with symptoms such as fever headache, confusion, stiff painful neck etc they should go to the er now.
Very. Depends upon the type of meningitis, in whom it occurs, how rapidly it is recognized and treated appropriately, etc. But in general it is highly lethal untreated.