A mask like face (lack of expression) can occur if you are aggressive and over-treat different areas on the face.
This is an outdated mode in using Botox®. As long as you dont inject more than 2 areas under an expert, the results are very natural.
Botox® lasts 8 months, repeated injection will prolong its effect. However some people metabolite Botox® quickly, making it last around 3 months only.
When injecting too little, the effect will also be short lived.