Mortar quantity depends on the unit size and joint width. Block uses more per square foot of wall than brick because the joints are thicker relative to the number of units, but fewer bags per unit.
Pre-mixed or site-mixed
Bagged mortar mix already contains sand and only needs water. It is more expensive per cubic foot and far more convenient on a small job.
Masonry cement plus sand, mixed roughly one to three, is what most production masons use. It needs a mixer and a sand pile but costs much less at volume.
Mortar types
Type N is the general-purpose mix for above-grade veneer and most residential work. Type S is stronger and used below grade and for structural block.
Type M is stronger again and used for foundations and retaining walls. Stronger is not automatically better — mortar harder than the brick transfers stress into the units and cracks them instead of the joint.
Tooling the joints
Joints get tooled when the mortar is thumbprint hard — firm enough to hold an impression but not yet set. Too early and it smears; too late and it crumbles.
A concave tooled joint compacts the surface and sheds water. Raked and struck joints look good and hold water, which is why they fail first in freezing climates.
Mixing water and retempering
Mortar is mixed to a workable consistency that holds its shape on a trowel, and it stiffens as it stands. Adding a little water and remixing — retempering — is acceptable within the first hour or two.
Beyond that, the cement has begun to hydrate and adding water breaks the bonds already forming. Mortar retempered too late looks fine and cures weak.
The working rule is two and a half hours from mixing in normal weather, less when it is hot. Mix what the crew can lay in that window rather than a full batch at the start of the day.
Weather is a hard constraint
Mortar needs temperatures above about forty degrees to cure properly, and it needs to stay there for a day or more. Mortar that freezes before it sets is ruined and the wall has to come down.
Heat is the other end of the problem. In hot, dry, windy conditions the water leaves the mortar faster than the cement can use it, and the bond is weak even though it looks set.
Cover new work, keep the units damp in hot weather and dry in cold, and do not try to push a masonry job through a cold snap.