I'm not a expert on HubPages by any means but my advice would be to take it month by month for the first 4-6 months, see how things pan out. Don't set any targets until you've got an established core of hubs, a name to go with them and some sort of strategy you can stick to. Say, commit yourself to a hub a week, maybe two. It all depends on how much time you can spend fully focused on this site.
If you can write hubs that are popular and can sustain quality, after 6 months perhaps then you could allow yourself to go for a minimum monthly income and not disappoint yourself too much if you don't achieve it on a regular basis.