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This is not an assumption concerning cost behavior.
General and administrative costs may be either fixed or variable.
"Linear" means that when a function is graphed, the relationship between the X value and the Y value will graph as a straight line. The cost function is assumed to be linear. Thus, when cost and level of activity are plotted on a graph, the result will be a straight line. The straight line begins at the level of fixed costs on the Y axis (where the activity level is zero) and moves upward by the amount of the variable cost per unit for each one-unit increase in activity on the X axis.
This is not an assumption concerning cost behavior. The time period used can be any time period management wants analyzed.