What is the requirement about invitation letter for China tourist visa application?

Actually, China tourist visa is valid for 3 months from issue date with duration of 30 days. That is to say it is ok if you will get the visa in July. Visa officers will take into consideration as what is written in your invitation letter. If your invitation letter is from a travel agency, it should better be original, invitation letter by fax might not be sufficient if visa officers deem that. China tourist visa is usually single entry, it allows you to enter China for one time during its validity. If you planed to enter China twice, you can try to apply for a double entry visa, or multiple entry visa if you need.