I've used ultraseal along with co2 cartridges and got it back home no problems after a puncture, well worth it but tyre fitters don't like it because of the mess left in the tyre when they change it but who cares if it does the job.
the ultraseal doesn't make the tyre go down quickly so you have more control over the bike if it does happen.
It's meant to keep the pressure up on the tyre and use centrifugal force to push the stuff to the puncture but on the occasions of my punctures it's stopped the tyre going down quickly, then I've just pulled the nail or cause of the puncture, blew up the tyre using 2 co2 cartridges and that's been it, obviously checking pressure regularly after but found the repairs to last till I replaced the tyres.
Another question on the cartridges, how many does it take to get enough air(or whatever is in the carts) in the tyre
I've always used upto two on a motorbike tyre and one on a push bike.
And in answer to your previous question....they should last forever as if they leaked after years they wouldn't be much good as a sealed cartridge
