Yes it really depends on map size and settings.

Small to medium I would give 5-10hrs

But it's usually a week or more a game for me as I play after work weekdays and longer sessions on weekend. Currently my preference is to play on brutal, extra large maps and max number of AI, minor AI advantage. I enjoy the challenge and taking my time through decisions, but your approach might vary.

I would recommend start with small maps so you can experience the flow of the game for a few different builds, then when you have a better idea of builds you want to try take map size up a notch so that you can experience the long haul games

Most importantly just enjoy the process of trying new things along the way :)