It depends to some extent on the blog, the subject matter, area of specialty and how interested you are in the subject yourself. Some of my blogs lend themselves to more frequent posting than others. If you can do it, and I hope you can, a daily post will keep readers coming back to see what's new. If you can post a couple more times on some days, even better.
How to do this?
You need to build a backlog of ideas and subject areas to explore and develop, an ongoing collection of links, and sources for news items your readers may not find without you. Then you will have many items on your back burner, incubating and developing while you write what's most interesting to you, most timely, or what you've picked out of your idea and info barrel because it's the best option you have for today.
Keep a notebook with you, a smallish one that fits in pocket, purse, or tote so you can write down ideas for posts and series of posts, facts, questions, and insights that come to you. No matter how brief, your notes will capture fleeting thoughts and opportunities that come your way that you can develop later.
On your computer, set up a one or more places for notes and links. If you have a Mac you might put them on Stickie notes, or create a simple text document to add them to, leaving it on your desktop for quick access. And/or use a blank email message, one without the TO: field filled in, and label it so you can find it easily. Paste your links into the message body, write your notes there, and save it in your Out box. It isn't going anywhere because it has no email address.
Whatever media you use for it, make your notes and links easily and quickly accessed so you will be able to add to them and retrieve them instantly.
What about when you haven't posted for awhile-when visitor stats dwindle and you wonder if previous readers will ever come back again?
Don't apologize for not posting. Hardly anyone wants to visit your blog to read why you've not been posting. Just begin posting useful stuff again. And market your blog to bring those stats back up to where they were, and beyond.