I think you will find this really varies from person to person and right now even experienced hubbers are struggling as their traffic has recently fallen off a cliff. http://hubpages.com/success/ - Check out the graphs on the success stories to see what I mean.
To earn $250 a month will be different from hubber to hubber depending on a number of factors such as:
1. number of hubs written
2. keywords targetted in hubs
3. competition for these keywords/hubs in the google - which will affect where you are ranked in google
3. traffic to each hub
4. HP Adprogram or just Google Adsense. (I'd suggest you sign up for the HP Adprogram)
5. evergreen content
6. how much you promote your hubs
7. the average cpm on your account
You will have to decide also how you plan to try and earn your $250 per month.
Are you going to go for lower competition terms that have lots of trafffic so it's easier to get ranked on google, but will probably result in lower CPM's. Or will you go for more competitive terms, with higher click values and CPM and much lower traffic.
Will you try and earn extra revenue through amazon and ebay links?
I have 70 hubs and will probably earn $200 this month from the HP Adprogram. I have approximately 90k views for the month and have perhaps made a few extra dollars if you add in amazon and adsense revenue for the month (I don't have any ebay links).
However, I have been told that this would appear to be a higher return per hub than normal. The yardstick I was given at the beginning of my HP adventure was to aim for each hub to make $1 per month. On that basis you'd need 250 hubs. Even if you manage to get them to make $2 p/m that's 125 hubs and so on.
For your own account, if you are using the HP Adprogram, the following calculation will show you how many hubs that you'll roughly need to make $250 p/m.
impressions x CPM = $250
250/CPM = impresssions needed
then you can surmise how many hubs you'll roughly need by figuring out the average impressions per hub you currently get.
It's important to remember that you'll need a lot more views than impressions as the impressions are the 60% you are paid on while the other 40% of views HP keeps the revenue from.
http://hubpages.com/community/A-Hubpages-Experimen... - Here's a hub I wrote about my personal experience on HP this year. I keep it as a sort of monthly journal to help me understand my journey so far on HP and other's really get an insight into HP and how it works. It will give you an idea of how easy/hard it is to make $250 p/m consistently from HP.