How is the amount of time for an A check or daily maintenance determined?
This answer is not specific for A-checks, but covers the whole maintenance effort.

Try Eddins-Earles, Mary: Factors, Formulas and Structures for Life Cycle Costing, Eddins-Earles, 89 Lee Drive, Concord MA 01742, USA, 1981

It's a bit dated, but has a wealth of data.

Another source would be Fabrycky, Wolter J.; Blanchard, Benjamin S.: Life-Cycle Cost and Economic Analysis, Prentice-Hall International. ISBN 0135383234.

Also, the white volume (I think it is No 8) of Jan Roskam's collection is a good start. Roskam, Dr. Jan: Airplane Design, Roskam Aviation and Engineering Corporation, Ottawa, Kansas 66067, USA, 1990.

Generally, this is not exact science, so exact times cannot be expected. With experience, each operator will be able to estimate how long things take, but you do these checks to find possible damage which needs to be taken care of. A lot depends on the training and experience of the maintenance crew. The best way to approach this is with statistics.