In a conventional gas / oil field, the hydrocarbons have migrated from a source rock and accumulated in a large porous reservoir over millions of years. You drill your wells and it either comes out under its own pressure, or you suck it out, until you've recovered all that is possible to with today's technology, which may be some time - the largest oil field in the world (Ghawar Field in Saudi Arabia) has produced 65 billion barrels since 1951 and is still going.
In a shale gas / oil field, the hydrocarbons are unable to migrate from the organic source material, because shale is impermeable, so they are adsorbed to it, building up in layers that may only be one or a few molecules thick. Drilling is done along layers that have high amounts of this organic carbon, or kerogen, in shales that have sufficient "brittleness" to be artificially fractured, which is why it's not suitable for every shale deposit. The more kerogen the well intercepts, and is able to be reached by fracking, the more hydrocarbons you produce, and the longer production will last. As someone said earlier, typically around three years. Multiple wells are drilled horizontally along these layers, and spaced apart so that the fractures from one just about meet the fractures from the ones around it in order to access as much kerogen as possible from as few wells as possible.
It's a bit like having a rain water tank that, once the time is taken to fill up, provides a lot of readily accessible water, compared with going onto your roof and sucking up the individual drops lying on it one by one.