How long does the sealant last/when will I need to top up? Also when I need to top up is there a limit on how many time I need to top up before I need to replace the inner tube?
yeah, make sure you buy inner tubes with removable valve cores for this, and then use leatherman pliers or a special valve core remover tool to add the sealant.
Yes.
Remove the valve core, squirt in the sealant, refit the core, inflate, ride.