If you're filling a new blank cart as opposed to refilling one, I always found it easiest to take out the wadding and fill the cart about 2/3rds full, then ease the wadding back in with a pin or something similar...then top up with about 5 drops until the wadding is moist but not pooling liquid on the top. That ensures the wadding is thoroughly saturated and you won't get a dry or burnt hit from it. Afterwards, you only need three or four drops to top up each time if you're using the same flavour e-liquid.
How long it will last between top-ups will depend on how fast you vape it and whether you put it down at all!
If you mean how long will a cart last...you can take out the wadding and rinse it under the tap, dry it by pressing it in a tissue. You can rinse the cart too, make sure it's dry before refilling as above, always do this if you're changing flavours. The cart itself is virtually indestructible, but the wadding will get tired eventually and can be replaced with the kind of Fluval used in fishtank filters. If you need some of that at any time, send me a message and I'll send you a ton of it... I prefer to use tanks now instead of cartridges as they're easier to handle, last longer between fills and are more economical on juice.