When we say "I'll be back in XXX minutes/hours/days", we mean that that is the latest we will be back. We might be back earlier. If someone said "I'll be back in about 30 minutes" and then returned in 40 minutes, that would fall under the definition of "about 30 minutes".
My opinion on this is, of course, coloured by the fact that I am extremely punctual and people being late drives me mad. This caused great amusement to my Spanish friends when I lived in Madrid. We would arrange to meet at, say, 9pm a a local bar. I would get there at 8.55. They would roll up any time from about 10.15pm onwards and be baffled as to why I was sitting tapping my foot, looking at my watch and being generally a bit grumpy. I did at least stop texting them asking where they were, checking they were OK, asking if I'd maybe got the wrong evening etc. It was pointless anyway because they never replied to those texts.