It honestly depends on multiple factors of your spell:
1.) What the original intent of your spell was; if you cast a spell to make it rain, once it rains, your spell has completed its course and is over. If you intended for your spell to have a larger and more spacious effect, such as casting a binding charm to bind an enemy or someone of the like from doing harm, then most likely its effects will last for a while after completion.
2.) The overall strength, energy, and will that you gave your spell during casting. If you cast it half-assed without really giving two hoots about actually seeing that it works and manifests in your life, then you can be sure that if it did ever manifest and reach completion, its effects weren't long-lived. The more power and energy you invest in your Magickal works, the stronger all around they are in the long run.
There are of course other factors that play into it as well, but they unfortunately escape my memory off-hand right now. But those I mentioned are definitely the top 2 to remember and work on.
Also, as far as talking about your spells to others, especially Witches, is a dodgy subject for a few reasons. I personally do not like to mention to another Witch or anyone else the Magick I've most recently done because believe it - and like it - or not, their thoughts and personal feelings about said Magick once it's in their minds and consciousness can actually adversely affect the results and manifestation of your Magick. I've had and seen it done to myself a numerous amount of times, which is when I finally learned that my Magick is mine alone to ponder. I do not recommend it, unless of course you absolutely ethically and/or morally have to tell them that you've done a spell, ,aybe because it involves them or it is for them, etc.