How long can you keep black olives after opening?!
Preserve in sterile jar(s) in a saline solution or vinegar mixture as in the usual recipes, adding aromatic herbs, garlic, lemon pieces to taste and with a 5mm layer of olive oil.
If you refrigerate them, about 2 weeks. If you refrigerate them in covered glass or air-tight plastic container and submerged them in brine, then they will last for longer.