Probably one training cycle. This period of time should be sufficient to determine if you are on the right track. Even a failure can have positive benefits. You have eliminated, with a high degree of certainty, one training method or technique that you thought may benefit you. If you narrow your stance, you may require more back strength, as well as more ab strength as in the wide stance.
Narrow squatters tend to use more back while wide squatters use more in the way of hip strength.
Your assistance training should be geared to accomplish these new strength needs. Use moderate weights until you get the new stance down pat. Going full-speed into a new stance could result in injury and loss of confidence. Another common question is leaning forward during execution of the squat. The result is usually either registering a lower lift or a missed lift due to depth problems.