The saying "quality over quantity" comes to mind. Even the smallest map (which would be a few rooms+halls) should take (a minimum of) ~5-10 hours of work. Anything less than that and it will most likely not be very detailed or interesting. That's not taking scripting into consideration.
If you do plan to take your time with 150+ maps, it would take about 750 hours with many small maps, which is about a month (keep in mind that's a month worth of your free time to put towards this, realistically that could be up to a year or two.) If you plan on having large maps, double that.
Honestly, I feel very skeptical about this. I feel that it will either never be finished, or finished with low quality. I hope you can prove me wrong, I'm tired of boring / poorly detailed custom stories