12% of the original time period stated.
I agree with this.
It seems related to the next smaller common unit of time. “A month or so” means 1 month +/- a week. Because if it was much more you’d say “about 6 weeks” or similar.
But this is a pretty informal term, so I doubt that there is a commonly agreed ‘definition’.
Oh, for GOD’s sake, you just want to bang on their heads with a hammer! And stir the pasta now and then! And pay no attention to your mother: the salt goes in at the start, just like the instructions say.
I would say it is equal to the initial unit of time +/- 25%. So a month or so is equal to 3-5 weeks, an hour or so is equal to 45-75 minutes. If it is a time estimate from a general contractor double all estimates for time or cost.
In software development, “a day or so” means “sometime in the next few weeks”
I’ve always had the impression that it was just a ‘skosh’ more than a smidgen, but less than a tad. YMMV 
(bolding mine)
Speaking as a General Contractor with over 25 years of experience, double the cost and triple the time is just a ‘skosh’ more accurate, IMHO. 