I think it depends on the plot and how well the author has everything unfold. It has to be an easy read too, which means little grammar and spelling issues and no long run on sentences etc (essentials of good writing). One of the first stories I read here was about 70 chapters but I never got bored. It didn't have a complicated plot either. But it got me hooked to AFF and it's still one of my favorites. I've also read some shorter stories with a slightly complicated plot but the author was able to weave everything in and end it beautifully. Be sure to know how you want the story to start and end so you are not 'caught' by the plots and confuse the readers.

It depends on what you mean by complicated plot, really. I don't mind longer stories if the plot is interesting. The first long story I've read here on AFF was 73 chapters and I finished it in one day because I thought the plot was interesting. It was also those types of complicated stories. I feel like if you try to over complicate a simple plot it won't be interesting. If you're for example talking about a spy that is given a role to protect this guy until the government can kill him and then they fall in love and they are trying to escape, a long story would be interesting because it'll probably be a slow build.
But yeah, just don't overdo the plot. I've read so many "complicated" stories here on AFF and sometimes I'm just there wondering if the author even knows where the story is heading themselves because they're overdoing it making it difficult for the reader to understand.