Constant MAX_FORMAT_CODE_CHARS 

pub const MAX_FORMAT_CODE_CHARS: usize = 255;

Longest number format code, in characters (not bytes), that is parsed: Excel’s own limit. A longer code is invalid: NumberFormat::parse returns a General format (the cell shows its value unformatted; the stored code is kept as is) and TEXT returns #VALUE!. The cap also bounds every allocation that scales with the number of placeholders in a code.