I am in the middle of teaching a Reiki Level 1 class. Not only do I encourage the students to give themselves Reiki for 21 days after they receive their attunements, but I do that for myself right along with them. So you can do this after receiving an attunement or start for yourself at any time.
Why 21 days? Some research has shown it takes 21 days to form a habit. This is a nice way to begin to develop a regular Reiki practice for ourselves, just like meditation, yoga, or other exercise and energy work. And are we completely healed and good to go after 21 days? Of course not. One of the wonderful things about Reiki is that we will always have it and can use it on ourselves whenever we need it, because as my dad says, "Some days are diamonds and some days are stones." The most spiritually symbolic reason to offer yourself 21 days of healing is to honor Usui Sensei's time spent meditating on Mt. Kurama as he received Reiki.
It has been a good week to begin with the official start of spring by the equinox. This is the time of year, according to Chinese medicine, to focus on cleansing our liver, a big elimination organ. It's time to begin eating spring greens to cleanse our bodies, and begin spring cleaning to cleanse our homes. It's a great time to begin 21 days of Reiki for yourself to cleanse your physical, emotional, mental and spiritual energies. May the season renew and refresh you!