If some are coming from overseas as you have mentioned ... if it were me i would be giving them at least 3mths ... people will need to save for airfare & accomodation ... & they may want to make the most of the cost of their airfare & make a short holiday out of it therefore may need to give some notice to their employer, etc ...

I have an interstate friend who recently gave me 6wks notice for her engagement party ... it wasn't enough notice as i had already committed myself to another party & also i needed more time to save monies for the airfare, accomodation,... also DP to take time off work needed time to be organised, etc ...