This really depends on the way your paving is used.
Clearly, sealer on a patio will last longer than sealer on a driveway. Likewise, if your driveway has only light use with a small car leaving and returning once a day, your sealer will last much longer than if it has much heavier or commercial use.
On average, we would say that the sealer on block paving, pattern imprinted concrete, stone, slate, clay paviours etc will last in the region of two to four years before it needs some re-coating. However, you could well find that only the areas worn by vehicular traffic and foot traffic need re-coating as lightly used areas may still be in perfect condition. Our products are all designed so that it is easy to re-coat these worn areas.
Stripping of the surface is NOT required before recoating. InvisiSeal is different as it fully penetrates the surface, and, therefore lasts almost indefinitely. The application instructions in the Download section give full details of durability and maintenance.