The answer to this question will depend on your own unique situation.
Someone already in a generally well-balanced state who simply wants to maintain their well-being might desire only an occasional treatment, while someone facing a serious health crisis might find it more appropriate to have several sessions very close to one another and then reassess their situation.
Of course always keep in mind that for serious conditions of any kind, Reiki is best considered a complimentary treatment and should be used in conjunction with the appropriate type of primary health care rather than as a replacement for it. Because Reiki is safe, has no known contraindications and compliments any other type of care, you can receive Reiki treatments along with your primary care knowing that it will only enhance and never hinder your journey towards a healthy balanced state.
You can never overdose on Reiki as it simple stops flowing once the recipient was drawn all that they need at any given time.
When you are deciding how much Reiki to receive and how often, I will be happy to share my thoughts on your situation after your first treatment (only afterwards, because it is through treatment that I am able to assess your situation). However the key is to listen to your own inner guidance and decide what feels correct for you based on all the factors operating in your life at the time.