how long does it take to lose a stone in weight

[c=665D00][i]unfortunately i have put on a stone, i lost nearly 4 stone years ago through diet and exercise, because i got very fat and contracted diabetes type 2....but now i've gained a stone back which i feel disheartened about....i go to the gym near me to swim for an hour, a couple times a week...how long will it take me to lose the stone in weight if i really try?[/i][/c]