Slate roofs can last decades without the need for a re-roof. the steel nails holding the slate in position will fail long before any sign of slate erosion becomes a problem.
On ocassion slates will slip down a roof, causing rainwater to penetrate the building. This may simply be due to incorrect fixing of the slate and can quite easily be fixed back into position. With a bit of maintenance like this you can prolong the lifespan of your roof and stop any damage occurring to ceilings and the roof timbers on your property.
Sometimes, as i’ve pointed out, the steel nails holding your slate in position may have begun to fail. If this is the case, then the roof will sooner or later need to be replaced.
There is no need to panic at this stage, envisaging a total replacement of everything on your roof.