Curing time for a various grade of concrete is described in ACI 308( Guides of concrete curing).
Definition: curing is the process of maintaining the temperature and moisture condition of concrete for hydration reaction to work normally so that concrete becomes hard and strong.
43 grade and 53 Grade Cement is used for large and rapid construction. Because is grade of cement has a rapid setting and hardening properties.
Mixing M sand or natural sand does not make any effect on strength of concrete. Both acts similarly. Also, it's curing period is 28 days.
You may also refer IS Mix design for, time and duration of curing for a various grade of concrete.