Mach 1
Of course it varies. (Speed of sound gets FASTER with lower pressure, lower with higher temperature). It also depends on mean molecular density (higher on Mars - CO2 (mass of main isotope = 44) than Earth - N2 (mass of main isotope = 28) + O2 (mass of main isotope = 32)...)
The important thing is that at sub-sonic speeds, one can deploy trim and drag panels and 'chutes without any problem other than strength and direct aerodynamic effects
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Above the speed of sound, shockwaves are involved, with the subsequent issue of machinery or fuselage damage,vibration, higher drag (normally good, but not ALWAYS), and severe aerodynamic effects.