What is considered good length for spin bowling and where do you measure that distance from? I am assuming from the wickets and about 4 to 6 meters.
I've started quite late to cricket in general, nevermind starting out as a spinner. Here's what has worked for me so far and I assume it's mostly the same for any type of bowler.
Time in the nets. Not only with mates but also on my own. That and looking where I want to put the ball has helped me in getting about the length I want. The approx length depends on what I'm trying to do, cramp him, aim at the wicket or get him to play to the field. No guarantees of course but it's good to have a plan. Using red/white balls and filming what I am trying to do with the ball and what the ball actually does helps in getting the gap between the two to be smaller. Vary your pace and position on the crease so you're disturbing his rhythm/timing without disturbing yours. Don't vary it too much though. Was told to 'always keep your arm as high as possible on release' then realising I can bowl spin with my arm being lower causing a flatter/turning delivery which is quite handy on bouncy wickets. It's not above their eyeline but it can threaten their wicket or at least make them think. For batsmen who have good/quick footwork, it's normally your stock ball or an awful delivery that gets them out (if you're lucky). Don't ignore that voice in your head.
Accept that even your best ball is going to get hit. It doesn't matter if all you can do is admire the shot. The next ball, the stakes are still about the same for both of you.
I did get a training session from a proper spinner but I think due to my age (old dogs etc.), the most I got out of the session was more confidence.
That's my take on things. I've found it to be a great learning curve.