When mortar is set to the point that it is "chunky", it should be pitched.
   Plaster brown mortar can't be re tempered, and has to be pitched. As a 
plasterer, we learn methods to extend the working time of plaster,
such as using clean water to mix with. Dirty water makes plaster set faster,
and does the same for cement mortar. It hurts me to see bricklayers wash
their shovels, etc. in the same water they mix with. Their mortar has to be
re tempered often to maintain workability, not only weakening the mortar,
but it is a complete waste of time. Time equals money, remember?

Portland cement mortar can be easily scraped off with a finger.
Mortar in the stone work on this multi-million dollar house was 
constantly re tempered
Certain rules should be followed, extending working time, to avoid re tempering.

1. Use clean water

Water should be clean and clear, even drinkable. Well water usually makes mortar set faster than city water, due to the minerals.

2. Don't let the mixer run too long, or mix by hand.

The longer the mixer runs, the faster the mortar sets. The mixer should run barely long enough to mix the mortar. The mortar man should shut it off while he does  other chores. Mixing with a hoe is better. The mortar can be dry mixed first, and then wet mixed, a little at a time, as it's needed.

3. Use clean sand.

Reddish sand indicates a high clay content. Clean sand should be yellow. Sand should be sand, and not dirt.

4. Work in the shade.
Work should be planned to allow as much to be done in the shade in hot weather.
       The mortar box and mortar can be easily shaded by throwing a piece of plywood
or cardboard over it. 

5. Use clean tools
Shovels, wheelbarrows, etc. should be cleaned every day. The old mortar gobbed on tools sucks the water and the life right out of the mortar.

6. When in doubt, throw it out.
Stiff mortar should be pitched when it is too hard too work with.