I'm using the following rule of thumb:

• If I like the concept and wanna "see what happens", typically 0,5 to 1 ETH
• For projects with good team, product and user base generally 2,5 to 5 ETH
• For projects I'm really bullish about (niches that I know), 5 ETH and above

These are my figures, but I know friends using the same proportion within their own budgets (some higher, some lower than mine). But in any case as new ICOs are popping up on a daily basis, its super important to stay cautious and do some research before funding the ICO. Try to diversify your investment across multiple projects and be prepared that you might suffer a heavy blow.