My favourite way to charge crystals is by putting them out in the sunlight; Sunlight re-charges crystals with strong, bright, positive energy.
The time it takes depends on how much the crystal has been used and I think , the intention of the person who has been handling the crystal.
Typically, I find an afternoon in the sun about once a month keeps an average sized crystal energetic and alert. However, some crystals require a longer time to recharge if it has been a long time since their last refill or they've been in a stressful environment, and you will need to do it more often if you carry them every day (don't forget to cleanse them first), just go with your gut felling.
Be warned that some crytals will fade in the sunlight, so if they are delicate in colour use common sense and only put them out for an hour or two at a time, and not so often.
The other most common ways to recharge your crystals is either putting them out in the moonlight, (very feminine energy, good if you can be an overbearing or aggressive person), or laying them on the earth (though be careful that the crystal is NOT water soluble if the earth is wet), great if you are someone who is a bit off with the fairies and needs grounding
Different crystals will prefer different treatment, have a chat with them, tell them you're going to put them out in sunlight and see how they feel, it might be you just find yourself thinking that you’ll leave it for now, then maybe wait for a full moon instead, just trust your instincts.. and take a moment when you bring them in to sense how different they feel, they love it.. and hopefully you will too.