If you're considering a blog for your firm, the first step is simply to understand why you would use the blog in the first place, as that can impact everything from the content you produce, to how often you produce it, to who you send it out to.
For instance, if your goal is to use your blog as a tool to help generate new business, it needs to have regular new content that is relevant to your audience—updating once a month is a bare minimum, but twice a month or even once a week is preferable (and the more frequent blog posts can then be consolidated into a monthly newsletter). The content should have actionable tips and guidance—material that people who aren't otherwise very familiar or engaged with your firm would be interested in reading and might share with others (especially in today's age of social media).
The content should be targeted at issues relevant to your clients' lives; if you're not sure what kind of content is relevant, that may be a sign that you need to take a step back and do a better job defining your target market in the first place. And perhaps most important, you should have a way on your website, and as a part of your general marketing process, to add people to the mailing list for your blog updates, so that you can occasionally push the content out to them, turning a one-time visitor to your website into an ongoing drip marketing prospect who may eventually become a client.
On the other hand, if your goal is to use the blog to communicate relevant news and updates on the firm to your existing clients, the content may be more irregular, as it's really only necessary when there's something to actually communicate. The content may or may not be actionable; it might simply be informative. It's likely not information you would communicate to prospects at all (at least, not all of it), but you should have a process to regularly distribute it out to your existing clients from time to time (e.g., once a month) so they're aware of both the content itself, and that your blog is the place to go for updates.
