It is important to collect the flower buds before they are fully open, usually during the last weeks of Spring. I've fermented some that were slightly open, but the taste differs and they result in softer capers.
If you want a stronger flavor you can leave them to ferment at room temperature for up to 15 days. Keep in mind that the more days you leave them to ferment out of the fridge, the stronger and more intense their flavor will be.