How long is a piece of string?
There are too many variables.
What are you goals?
What is your starting point?
How much time and effort can you commit to reaching your goals? And so on.
Everyone should be able to see change within 12 weeks, if not something is wrong. But seeing change and acheiving your goals are not the same thing. It may have taken you years to put weight on, so you can't expect to lose it all in a week or two. You need to be realistic.
Don't be limited by other peoples beliefs either.
12 weeks is a realistic time to see and feel change in anyone and should start you on the path to better taking care of your health and fitness, then continuing on towards your goals.
Everyone is different, so is their physiology and their journey to get where they are going.
If you want to make changes then get in touch and we'll discuss how you can acheive that.