On the B757 it will be weeks, not days and depending if it is a 4C check it might be months.
"C" check intervals are by numbers, the higher the number the bigger the check, so longer down time.
"C" checks on the B757 are normally every 18 months, so an example would be like this, 1C, next time 2C & 1C, then next 3C & 1C and next 4C, 2C & 1C and so on.
The normal numbering on the B757 is 1C, 2C, 3C, 4C, 5C, 6C, 8C, 10C & 12C.
Hope this is of help.
