With the SSI/Opinion Outpost survey panels, the general rule of thumb is that you can keep trying for surveys until you complete five surveys across their panels (example- a USA user could do 3 United, 1 Spirit, and 1 Jet Blue survey in a day or some other combination among those programs) or until they run out of surveys that potentially match your profile.

Weekdays generally have a decent number of surveys available and Sunday is the slowest survey day of the week.