Count'em In - Playing with Others Series
If someone else is the leader, they'll count the song in and look to each player as it's their turn to solo. When it's your turn, the common length of a solo is two choruses (24 bars), though you can play less, or more. If it's less, they may say "go again!," so you're best off to be ready to play two choruses. If you play more than two choruses you better use structured phrasing (Chorus Forms) to keep the listener's interest. Also be aware that in a jam situation it's rude to play too many choruses... use your best judgment. Keep your eyes up around each turnaround, this is where things happen (solos start or end, breaks, dynamic changes and the ending).