What is the minimum amount of time you can work for an employer and still get a (positive) recommendation from them?

I have a longer post I've been meaning to write, but the short of it is that my work environment has become unbearable (weird home office environment, inappropriate boss, no HR department). I am paid so little that each day is a struggle to show up. I just found a suitable job paying almost 40% more and am chomping at the bit to apply. The problem is that I was a NEET before this, so I don't have anyone else who can write a recommendation letter for me. How long would you stay for a recommendation?