Each airline sets up the criteria for each one of those checks in conjunction with the manufacturers recomendations, and their operational needs, the criteria is not set in stone. Some airlines do not use the A-D check system anymore.
For instance the airline that I worked for when they were using that system would accomplish an A check (overnight) at least once every 5 days. The B check every 3 months they usually took 1 or 2 days. The C check on a wide body every 4,000 flt hours, in anywhere from 20 to 30 days, and incorporate a 1/4 D check with each C. The C checks were accomplished every 14 months, on average, that way there was never the need for a D check. The D checks were supposidly to be done every 5 years on average.
"Saddest thing I ever witnessed....an airplane being scrapped"