How much does an average 25 year old save per month, and how much of his salary would this amount to be?

I think it really depends on the household and lifestyle demographic of the individual. And since these are qualitative aspects of a person, I don't think there's an average for it. For a fresh grad earning $3-4k and living by themself and footing all bills, it is likely that they'll be saving less than 50% of their income. As compared to someone who has a starting pay of $7k or more, they would be able to save a higher percentage of their salary - less any extravagant expenditures.