We all have those days where we may have had one or two (or three) extra pieces of cake at your niece's 4th birthday party. Does this mean that by morning the damage is done? After all a moment on the lips is a lifetime on the hips...
Fortunately, this is not quite how it works. In fact from what I've researched (and since I am no doctor), all I have found is that there is no real way to figure out how long it takes for excess calories to turn into fat. Of course there is a balancing act of calories in with calories out, but many other factors that impact the answer to that vary from person to person based on genetics, metabolism and hormonal factors.
Temporary weight gain could appear quickly after a large meal or two, but that does not mean that if you've overdone it, you're doomed. After all, daily weight fluctuations are normal.
Generally speaking though, if you keep your calories burned in line with your calories consumed over a week's time, you shouldn't gain -- 3,500 calories is equal to one pound. If you do have a surplus of 3,500 calories by the end of one week, most people will be up a pound from the prior week. The most worrisome is the gradual weight gain that people experience, which is more likely to represent new pounds that you truly gain over time. Keep a food diary to try and help combat this gradual weight gain and (you knew this was coming) exercise regularly!