Depended on the assembler. Some let you have 8 characters, internally uppercased and silently truncated. Others let you have 16 chars, others were effectively unlimited.

I wrote a bunch of assemblers, each one better than the last, ending in the one that Atari used for Atari ST, 7800 and Jaguar development. It's the assembler included in the 7800 tools archive. Someone cleaned it up a bunch and put it on github a few years ago.