It varies massively, but you should consider 65,000 words to be about the lower limit. I know it's ridiculous because it has to be the length it needs to be, but in the real world it actually has a 'fixed' sort of length.
Anything from 65,000 upwards to about 100,000+ is good. If you write anything noticeably longer than that and you're a first time writer, a publisher won't touch it - too log, too risky, too expensive, too everything (they're not in the risk-taking business).
Screenplays are the same, they have a set length, and if you deviate from that you won't even get it read, let alone taken up. A screenplay needs to be something like 90 - 110 pages long (a page equals about a minute of screen time). Longer than that would be seen as risky, expensive, not worth it.
Having said that (talking about the novel now), don't let me put you off doing what you want to do. Lots of experienced and successful novelists get more rejection letters than you'd imagine, but they believe in what they're doing and stick at it.
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