Before Ambitions, I use to let my sims reach level 10, become elderly and then retire them, unless they were working on their LTR. Now, with the new professions, kind of weird seeing elderly firefighters or someone in fashion. I usually get them to level 10 and elderly and then invite as many people over and have them retire. I like the little party they have when they retire. So I guess you could say, I like to have them go for as long as possible before they retire, but always have to reach level 10 in their career or profession.