An A-check takes between 24-48 hours and a C-check takes up to 1-2 weeks. That's the biggest difference, so an A-check is more like lubrication, boroscoping engines and servicing.
While in C-checks the bigger stuff is removed for inspection like flaps or the gears.
And you have also a D-check or also called a HMV(heavy maintenance visit) And this takes up to 4-6 weeks and there the aircraft is really taken apart.
The checks have an interval, this is also depending on the oprator, for an A-check of every 6-7 weeks, an C-check every 1.5-2 years and a D-check about every 6-years.
I hope this helped a bit
