Each airline is different, I can only tell you what we do.
A "service" check is done every 3 calendar days. Done on overnights, so no time out of service.
"A" checks are done every 110 hours. Six phases of "A" checks - A-1 thru A-6. Each phase incorporates a part of the "B" check, so we no longer do B checks. An A check takes about 48 hours.
"C" checks are done on a 20 month schedule, and takes 4-6 weeks. Each C check is around $1 million+, and each incorporates 1/4 of a "D" check, so we no longer do D checks. They are part of the C.