The PSK31 specification by ARRL specifies:
There is a preamble at the start of each transmission and a postamble at the end. The preamble is an idle signal of continuous zeroes, corresponding to continuous phase reversals at the symbol rate of 31.25 reversals/second. The postamble is just continuous unmodulated carrier, representing a series of logical ones. This makes it possible to use the presence or absence of the reversals to squelch the decoder so that the screen doesnt fill with noise when there is no signal.
How long are the preamble and postamble? If there's no specification of length, what's common practice?