On optifast I did it in about 4 months but it is a drastic weight loss plan. Limited foods and you feel unwell for at least 4 days as your body adjusts. It's best done with support of a dietitian. And it is expensive.
On weight watchers I am losing consistently minimum of .5kg a week, usually more. Even over Xmas / NY and birthday week I managed to not gain anything.
The more weight you need to lose the larger the losses initially. The cost for online is very affordable but many people enjoy and get a lot out of meetings.
Best of luck.
Kath x