If it's stored properly you have some time. Remember, lox is preserved with salt. This is how people kept fish from spoiling before the good old ice box. As with the other posts, your nose will tell you.
If it's fresh cut from a deli it's probably bad by now, but if it's a vacuum-sealed packet that's been kept refrigerated it may well still be fine. As others have said - open it and if it's bad your nose will know.