Now that I think about it, why is it normal to say "ten days to half a month" (十天半月)in Chinese?
My best guess is that it was common in the past to split each month into 3 periods (旬), i.e. the upper, middle, bottom periods of a month (it's still preserved in modern formal Chinese but not really super casual lingo). If someone roughly travelled from one period of the month and arrived during the next period of the month, the travelling time is probably around that duration, I guess?
Anyway, I don't think "ten days to a month" is the best translation, although it's correct literally, since it's used quite liberally for, and beyond, its implied flexibility.