How to insert a new line character after a fixed number of characters in a file

I am looking for a bash or sed script (preferably a one-liner) with which I can insert a new line character after a fixed number of characters in huge text file.

How about something like this? Change 20 is the number of characters before the newline, and temp.text is the file to replace in..

sed -e "s/.\{20\}/&\n/g" < temp.txt

Let N be a shell variable representing the count of characters after which you want a newline. If you want to continue the count accross lines:

perl -0xff -pe 's/(.{'$N'})/$1\n/sg' input

If you want to restart the count for each line, omit the -0xff argument.