The chain should be oiled regularly and the tension checked and adjusted periodically. Generally it gets slacker as it gradually stretches and the tension is increased by loosening the back wheel nut and then adjusting the small screws to move the wheel backwards slightly, increasing the tension. The sprockets wear over time too so should also be checked or the chain could slip. If the chain is well adjusted,clean and oiled then it and the sprockets will last a lot longer. The length of time it will last will vary depending on the bike, riding, mileage, quality of the chain and how well it is looked after and kept rust free. The less robust chains on a 125 will probably not last as long but hard to say as it is not propelling a 200kg bike with a 1000cc engine