Hello,
last I checked (way back on 3.x), this was technically only limited by the type of database you are using, since they have different max. lengths for textual columns. To be on the safe side, try using not more than 255 bytes (depending on character set / encoding, the amount of characters may be lower). But I know that we've had a silly 1000-character tag once on a customer test environment.
Regards
Axel