Well, the potential for every hub is different. It depends on the topic and how many people are searching for that topic. You can write a wonderful article about an obscure topic, rank #1 in Google, and get very little traffic because nobody is looking for that information. So, just because a hub doesn't pull in a certain target doesn't mean the hub is bad - it could mean that the keyword is bad.
Personally, I use two thresholds to determine how successful a hub is. The first is 100 views / month. If a hub gets less than 100 views a month, then it's just a flop - could be poorly optimized, could be a poor keyword, who knows. If a hub is getting around 100 views or slightly over, then it's ok. Definitely not a great earner, but worth keeping around.
The next threshold is 1,000 views / month. This is where I consider a hub to be successful and earning a decent amount of money. If you estimate that you earn about $3.00 CPM (after you give HubPages their 40% share), then 1,000 views per month will get you about $3 / month. On an annual basis, that's $20 to $25 per year. I'd be happy with that, and if a hub is getting near or over 1,000 views per month that I consider it successful.