You should be able to go at least 7 generations. The key is to use it fresh, within 2 weeks of harvesting. Otherwise you should probably make a starter.
When I buy a vial or smack pack I usually start out with a schedule of what I am going to brew the next several weeks. I may start with a dunkel, then go to a bock then a doppelbock or start off with an ESB then from there go EIPA, Robust Porter and Barley wine. The key is to start lower gravity and stay the same or go up. You probably don't want to reuse yeast from SGs much higher than 1.065, maybe 1.070.
If I am really lazy I may even start with a very low gravity beer, not make a starter and just pitch 2 packs of yeast per 5 gallons and use that as a big starter for my next beer.