Usually the only time I can smell it is when my nose is right above the airlock, sometimes when I walk into the room if the air is still after a few days but that's very subtle. The smell is a pleasant smell that for me has a sweet clean air/ozone smell which sounds a little weird but over all its just subtle smell of the good drink your making.
Generally when the bubbling stops for the most part sense that's when CO2 is done being produced from the active fermentation the better way is using a hydrometer and seeing when it reads the gravity of water, so about 1.000. But the other side of it is how long its left for the 'aging' process.
