How long does it take to become a Senior in your field ?

Actually, becoming senior it's not a matter of time it's more about the skills that you should develop.
A senior should be able to deal with complex tasks, master the product (especially the technical side for developers and the functional one for QAs), master the processes, become autonomous (need the least assistance from the expert members of the team), etc.
In my case, I became Senior Software Development after three years. During this period, I got the chance to work on several
projects (with different levels of complexity) which allowed me to gain experience and discover different areas of my product (MFACT)and acquire the required skills.
This is a brief description of how it works, I hope I answered your question.
If you need more details don't hesitate to ask me.