I can complete ME1 with DLC and every assignment in 18-20 hours. It would take close to 30 if I listened to all the dialog instead of just reading it or otherwise skipping it since I've gone through the story a hundred times. ME2 takes me 32-35. I have not played through ME3 since the first two times I did when the game first came out, so I never experienced any of its DLC.
The only Mass Effect game I skip the dialog while playing is Andromeda. You have such massive exposition dumps in that and some characters dialog can be upwards of 20 minutes long at a time. The Director Tan is the best example of this.