In reality the maximum path cannot be longer than 32 characters only !Sub-directories can be nested but the complete filename, including prefix must not be longer than 41 characters (note that if you are using a network device, for example n1_win1_proggy_bas, the maximum permitted filename length is reduced to 39 in current versions of the QL device drivers).
DATA_USE win1_1234567890_1234567890_12345
is a valid command
But
DATA_USE win1_1234567890_1234567890_123456
or
DATA_USE win1_1234567890_1234567890_12345_
will return "bad parameter"
Tested on QPC2, QEmulator, QL+SGC, QL+tetroid GC