Q.growing stock for cuttings
How far back do I cut the stock flowers to get the longest stem, but will promote reblooming? How long does the stock flower usually bloom for the season?
Depending on your growing zone, Stock may be an annual, biennial or a perennial.
The flowers can be cut at any length that you want for cut flowers, though you should remove the stem down to the crown of the plant.
Depending on the age of the plants and the environmental conditions you may have more flowering from the plants.
Here is a link with more information.