I am going to lodge a temporary visa named Recognised Graduate visa. My question is what is the starting date of its 18 months period, arrival date or grant date?
When I reseive the visa, how much time do I have to enter Australia?
Your visa will be valid for 18 months after the date you enter Australia, i.e. arrival date. That's valid for visas that have a fixed duration, e.g. working holiday (12 months).
Your grant letter will contain an enter by date, which means your first entry will have to be before that date. Therefore, you will have to enter at any date between the date of the grant notice and the enter by date.
To summarize some of the information in the wikipedia article on visas, and my experience in non-Australian countries, there are generally two different things referred to as visas in the process:
Generally speaking, to get the first document I need to submit my passport to the embassy or consulate of the country I want to enter, and they will attach a fancy looking document to my passport that tells me what the entry status would be, the duration of that status, the date of issue of the entry document, and the backstop date for when I am allowed to enter the country.
Before the first visa expires, I will fly in to the country, and present them my passport with the document attached. The immigration official will check the document in the system, and either affix a second document or stamp the existing document with the date the permission was granted, and the amount of time that permission is given for.
So in your case, the first document should tell you when you have to enter the country by, and upon entry you will be given 18 months from the date of entry on that visa. If you already have the entry permission document in your passport, you should check it, and if you have further questions you should contact the embassy/consulate to confirm.