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.