The Basis Refactor Frequency box on the Linear Solver tab:

allows you to control how frequently the linear solver refactors the basis matrix.  The options are either Solver Decides or some positive integer quantity.  If an integer value, N, is selected, then the linear solver will refactor every N iterations.  Numerically tough and/or poorly scaled models may benefit from more frequent refactoring.  However, refactoring too frequently will cause the solver to slow down.

The default setting for the Basis Refactor Frequency is Solver Decides, which will typically result in refactoring about once every 100 iterations.