The best time to defoliate the Ficus sp, in the Southern Hemisphere is from December to mid January. Although many figs are tropical, there are varieties for the sub tropics as well as the warm temperate.
The night-time temperatures are an important factor when used for bonsai. The more tropical may perform well enough in the ground in a frost-free environment, but because we prune and trim bonsai, the branch or branches may die-back if the night temperature fall below about 18 deg. This situation occurs with Ficus macrophylla the Brisbane, sub-tropical Moreton Bay Fig when grown in Sydney, the loss of a major branch can totally wreck a styled plant. The Benjamin group for instance responds well in the sub-tropics but is apt to self-defoliate in the warm Temperate Zone.
The reason most people remove the leaves is to reduce their size. Defoliating them at the start of the growing season, will result with as large or larger leaves as before, and is a waste of the plant energy.
Another little realised use for the removal of fig leaves is for refinement purpose. A refined ficus is one with uniform leaf size. The leaves are totally removed at the end of December, allowing enough time for regeneration before slowing down towards autumn.
In growing back their size will be similar. As the ensuing tip bud enlarges it is, in fact several leaves furled around each other. By twisting this shoot between the thumb and index finger, and removing about two leaves is slowing the growth. It is also shortening the internodal length and thus continuing the refinements procedure. The now, tiny exposed leaf becomes premature, for it would have opened out in about two weeks, will thus be more in uniform with the rest.
This information should only be used when the bonsai is well structured. That means the trunk appears think for the height and the branch ramification is adequate. Unless the above criteria are already present, in the long run, you will not be doing justice to the plant.