Hi 5sandown
You should find the information you need in this document: http://www.britishhedgehogs.org.uk/pdf/Hibernation-Weight.pdf
Bit basically, “…Those hibernating at less than 450g will probably not survive (although some occasionally do). .”
But for hogs which have been in captivity, they “… should not be released at weights below 500g in autumn (600g in very late autumn/early winter).”
That’s because they are likely to lose weight upon release.
If you’re fostering hogs I recommend that you read the whole document as well as the following:
http://www.britishhedgehogs.org.uk/wp-content/uploads/2018/09/BHPS-Guidance-For-relasing-Rehabilitated-Hedgehogs.pdf
and http://www.britishhedgehogs.org.uk/wp-content/uploads/2018/09/BHPS-Guidance-For-relasing-Rehabilitated-Hedgehogs.pdf and the link contained in it about releasing hogs over winter.
