How Long To Beat has cross-platform averages for Mass Effect and Mass Effect 2, shown below (although I would I consider these to be a bit high, it's all very dependant on the player, obviously):
Main Main Plus Completionist
Mass Effect 17:11:56 30:44:28 48:26:54
Mass Effect 2 26:09:34 36:48:34 47:19:13
Main is the barebones main quests, no side missions etc, so is what you're asking for.
However, Main Plus includes additional side missions, etc, so if you just play "normally" instead of speeding through (and you should, a lot of ME1 and ME2 side mission results are carried forward to influence the later games so if your aim is to play the series it'd be a waste to skip these) then this is probably more representative of what you need.