I created my site before I ever heard of adsense. I have since then put adsense on my site. I am not making a lot with it yet, but maybe someday I will. I have fun running the site and if down the line, I start to make decent money with adsense, then all the better. Guess it depends on why you created the site? Did you create a site covering a topic you really enjoy and put adsense on it in hopes to make it profitable as well? Or, did you make a site purely to make money from adsense? I think the answer to that question, will give you an idea as to when you should give up. Personally, I will continue you with my site even if adsense never becomes a big money maker for me.
Never give up. Just diversify. I make a little designing local sites. I make a lil more hosting all my clients sites, then a lil more editing and maintaning them, then I have various sites of my own, with adsence, linksales, booking affiliations. Nothing by itself amounts to crap but all together I get the bills paid and may have a hopeful future. Create lots of new ways to earn...