After you perform a spell, you can start looking around for sign from the universe, spirit guides or higher power that your spell is working. You can ask for a sign if you want, and see what happens.
I’ve learned through experience that the universe and spirit world usually start with very, very small signs like repeating numbers on a clock, a remark from a stranger, or an interesting coincidence. Basically, your first few signs will be things that no one but you would understand to be signs.
If you acknowledge and celebrate those signs, more will likely come. Usually, the signs will be bigger, flashier, and more difficult to explain away.
The new moon and full moons, as well as sabbats, equinoxes, or other annual celebrations, are excellent times to spellcast because these are times that are naturally more powerful. If you have any big spells to perform, you might want to take advantage of the special energy of a holiday or moon phase.
Besides that, you can spellcast anytime you feel mentally, physically, and emotionally up for it. No one will know but you if you are truly up for it. It’s ok to start a spell and feel too fatigued to continue. Stopping in the middle of spellcasting is much better than forcing yourself to finish and potentially cause yourself more fatigue.
As for performing the same spell twice, do this sparingly. It’s the energetic equivalent of you telling yourself that you don’t think your first spell worked (which is likely to negate BOTH spells!) Practice patience and cultivate a sense of trust in the power you called on to co-create with you. It will happen in the right way, at the right time.
If you strongly feel the need to perform the spell again, or you’ve decided to tweak your intention, those are appropriate reasons to repeat a spell. Use your best judgment, and you’ll be just fine.
Over time you’ll develop methods that work well for the kind of spells you typically perform but don’t be afraid to experiment with other forms of spellcasting. This can be particularly helpful if you feel like you’re in a rut with your magic. If you’re a solo witch, try spellcasting with a group. If you always use your own energy, try working with the elements. This intuitive exploration of magic will add a richness and depth to your practice.