Depending on the stress condition experienced by the solder joints. For joints under a greater stress, the time to reach equilibrium can be shorter. Of course, the change is also more significant in phase structure. For low stress conditions, it may take several years. For high stress conditions, it can be less than several months.
Yes, the material properties do change significantly. Usually, the change is toward a larger grain size, hence a joint more prone to fail.
For In/Sn solder, the solder exhibits superplastic behavior in creep, and has a longer isothermal fatigue life than non-superplastic materials. There is no microstructure change like coarsening and recrystallization that occur in Sn63.