I'm pretty sure the previous recommendations by the ADA and AAOS was for life, but I have heard differing time frames from other professional (surgeons and dentists alike). If I recall correctly, the orginal study in the 70s that created the recommendation for prophylactic antibiotics for dental work, had very little statistical value. The recommendation was more based on the potential for the problem to be really bad (ie revision), but the potential for the problem to even exist was super low. With the negative potential effect being so great, the recommendation was to simply take the antibiotics. In 2012/13, we may be a little more concerned about over use of antibiotics than we were in the 70s and thus I would expect that there are some new considerations being looked at.
The few times I've taken the anitbiotic, my digestive system gets messed up for a a week or so and that's with an increase in acidopholous. Based on the recent recommendation, I think I might steer away from taking propho anitbiotics.