Note that many choose to use the term "Virtual Professional" because there are some who feel "Virtual Assistant" gives a subordinate image.
So, if you'd like to work 30 hours, your projected revenue would be your hourly rate times 30 hours, times 4 to get a monthly figure.
Keep in mind that you will not start with 30 hours; it will take a while to build a client base.
You'll need to "reach out" and make it happen by networking and marketing (both in person and via the internet), and perhaps even by making phone calls to get your business off the ground swiftly.
To network, find a niche and join one or more associations or internet groups.