I believe the only boss in heroics that has an enrage timer is Eck. It depends on which of the following runs out first, your healers mana or the bosses hp. Before triumphs dropped in heroics I downed the first boss in HoL with a prot pally and a disc priest. That's seven minutes of my life I'll never get back.

The ICC 5 mans will be tough for you atm, however the 3.0 heroics shouldn't be a problem.