You need to start it when you don't think you need it. You'll really benefit from it only when your situation is totally, inconceivably different from when you initiated it.
This is my thought as I read a neighbor's blog. She started it, wrote about her kids and her life, so that then when her husband got a girlfriend she was able to talk.
I started this blog in a flurry one day, but realized after a week of tinkering that I had a clear idea of an audience but nothing worth telling them. My life was already fallen apart with no sign of light. My online diary would have read:
This really sucks.
This totally sucks.
OMG it's the weekend already, and everything still totally sucks.
So I say, never start a blog when you are going through something intense. Write when things are peaceful and you can set aside some time. Then when your world falls apart, you don't have to commit to anything big except survival.
More important -- talk to your loved ones about scary stuff now. Don't be afraid of life insurance. Give your honey permission to love again. If you get a lump somewhere, you'll never be able to deal with these things and they will suffer after you (whether the lump gets you or not).
At the first meeting of my young widow/ers group, I found out I was the only one who had talked about remarriage with their spouse. We had only done that because he'd had a previous health scare. After he got sick -- really sick -- it was too threatening to both of us to do anything concrete. No social security, no life insurance, a lot of things were much more difficult for we survivors. During the almost-two-years he spent dying, we had that magical thinking where we believed that thoughts could make things happen, but our plans could not do anything.
You never know when your story will end. You don't have to be cautious, enjoy your risks as you wish to, but be responsible and generous too. A word to the wise.
So while I was blogless (or publishing a blank blog) for a year and a half, I was sharing in many other ways (believe you me, many). I'll try to go back and recreate some of the missing entries 'cause I'm sure there's some insight in there (plus the wacky dating stories).
You -- DO IT NOW.