It's still up in the air as to what skipping ALL loyalty achievements will do, some say it'll result in Shepard dying every single time, while others don't.
To be on the safe side I did everyone's Loyalty missions (well that and I couldn't bear seeing a SINGLE person die) but maybe you can get away with just doing the big three (Miranda, Jacob, and Mordus) and still survive but it's all still guess work.
Doing all the Loyalty missions and beating Insanity took me a hair under 30 hours, you could probably cut 10 hours out of that skipping most of the Loyalty missions. But you'll still have to spend time mining planets for materials so you can make all the upgrades on your ship or else things can go horribly bad there.
Last edited by YouStayClassy; 02-23-2010 at 10:14 AM..