Under about eight words. Anything longer stops being a sign and becomes something people have to stand and read, which kills the timing. If the joke needs a setup, split it across two lines with the punchline in larger type, and drop every word that is not doing work.
.jpg?x-oss-process=image/resize,m_fill,w_640,h_480,limit_0/interlace,1/format,webp/auto-orient,1)
