As an electrical engineer, should I study pure mathematics in my spare time or do a second bachelor degree? EDIT: Short version: Can a full time engineer ever hope to learn pure math up to masters level on weekends in less than six years without burning out mentally or is full time study required to achieve this goal? 

I am an electrical engineer who has 100% decided to learn pure mathematics properly so as to expand my mind. Should I do this in my spare time or quit my job and do a second degree in pure math?

I'm 25, been designing hardware for two years and have learnt a lot related to electronics/magnetics/control theory/ embedded systems programming etc. I've reached a point where work is becoming very repetitive and I feel my brain is slowly dying. However, there are still small crumbs of very valuable experiential knowledge to learn as I do design after design. My salary will also significantly improve as I transition from junior to mid-level if I stay in my industry.

Is it possible to learn pure math properly in my spare time or should I dedicate 100% of my energy to it? If I do it in my spare time, can I hope to reach mastery in a reasonable timeframe (5 years), or is the nature and volume of the knowledge such that it requires full time attention? I don't know the extent of what I don't know. I also don't want to burn out.

Thanks
From your question, I assume that you are NOT interested in taking a degree in mathematics because you need to prove to somebody that you have a qualification. What follows is based on that assumption.

Learning 'pure math' is not the same thing as learning a language: pure mathematics is a myriad of languages. A bachelor's degree course in pure mathematics traditionally covers, shallowly, a very wide range of topics, most of which the students will never use again, even if they pursue careers as full-time professional mathematicians. 

I strongly suggest to you that if you need a better understanding of some aspect of pure mathematics for some particular purpose, then you should study just that aspect of pure mathematics. You may need to rewind a bit to find a starting point that you do understand. And you may need to broaden out your reading as you go along. 

If you have no particular purpose in mind, then just pick something that you are interested in. Do not expect to understand everything immediately. Do not be afraid to pick up non-academic 'popular science' type books. There are so many entry points that I am sure you will be able to find one that suits you.