I think that many of us have been through concious "image phases" in our lives... that's why we are on this board.
Speaking for myself, I gradually changed through the phases: Skater/Punk, Stoner/Hippie, Pseudo-Preppie, to professional. These phases also correlated with my friends and jobs. I know, this is all pretty much common sense.
But to answer your question directly, from the conscious decision to final metamorphoses of each phase took months to years. Most were fairly simple for myself, as each phase was not very drastic from the other.
I would gradually replace a few items as others wore out, or if money was available, several (5-10) items in a short period. I never threw out my whole closet and started over, and was in a constant state of weeding out clothes that I no longer wore. Having lower standards in the beginning was easier for the rapid/mass accumulation of wardrobes, as thrift store shopping was cheap.
My last change in social image was the most expensive, as I was forced to dress professionally in a mannner consistent with my chosen profession. This required a mass purchase of dress shirts (6), slacks (5), and ties (10). Again, I am constantly adding to my current wardrobe, and weeding out old clothes to make space for the new (especially after frequenting this forum). I do think however, that this will be the last "major" phase in my image- although allowing for constant refinement.
As we have both mentioned, the social circles that you associate with are a major force in directing one's image metamorphoses. For example, many of the members on here will post up an image of themselves or an article of clothing, willing to subject themselves to criticism/applaud. When these images are reinforced/rejected, it is natural to change towards the image that gives us the most positive reinforcement. Although many people say that they dress for themselves, "I believe", as in The Matrix
, that we all have our Ideal/Projected Image or whatever they called it in the movie. Is this Ideal Image concieved before we enter our chosen environment, or after?; That is the question...
Sorry for the dissertation.