As with a lot of things this is a bit like asking, how long is a piece of string.
There are many factors that determine how frequently the paving will need recoating, the biggest of these is how frequently the paving is driven/walked on.
The absolute shortest period the paving should need recoating is 12 – 18 months, this is with very heavy usage ie. a fully loaded van driving across it two or three times a day, but the chances are that it will last much longer.
Yes, if you use a solvent based sealer, and you must ensure there is very good ventilation as there is a stong solvent smell, which some people can find unpleasant. Alternatively use a water based sealer which is almost smell free