What criteria would you assess to gauge whether to quit a Ph.D for a job in industry? Just started my second year of a 3.5 year EPSRC funded Ph.D (in engineering, and using some machine learning) in the UK. I have recently finished my confirmation review so have secured the funding for the remainder of the Ph.D. 

I have a conference paper from my first year, but I have spent most of the year up and down with the ebbs and flows of research and I am not sure I can handle it mentally anymore. 

I am looking at applying for data science/analyst positions. I think I am far better suited to a life in industry.

Part of me thinks I should just try and grind it out and finish (maybe this is second year blues?), but part of me also just wants to get out and start progressing with my career (I already have a Masters degree). 

At what stage would you consider leaving a PhD in order to pursue an industry career?
"you think" you'd be happy working might not be enough, depend on the situation I suggest you to take a leave of absence (usually universities in the UK allow this) for 6-12 months, and get a job so you know for sure what ticks for you, then decide whether to return to your program or keep the industry position. Don't feel guilty for taking options that are readily available. 

I had similar experience and this, to go or to stay, might be the hardest question I had to answer myself for the longest time. In the end I decided to stick it out, but I also have a close friend who's having the time of their live ditching their PhD 2 years in. You just need to work out the best approach for your situation, and the best way to do it is from a little bit of distance. 

Good luck!