I'd say for a couple years experience, go with around 200 to 250 karma. It would depend on how often your characters ran of course, but the general idea is once a month I think. Missions sets 250 karma as a prime runner, though having played characters with more (as much as 600 karma), I don't find it to be too unbalancing or over powered so long as the GM accounts for that power. One way to do so is to use the suggested karma and BP values for Prime Runners found in SR4A. This gives examples of how much karma and BP an enemy should have compared to the threat they represent and how much the players have. Doesn't work for mooks but mooks are just mooks. Really a character can't even qualify for Street Legend until he hits 600 karma. (500 for Street Legend and then another 100 to buy that one optional power that turns all 4s, 5s, and 6s into successes).
Beyond the karma though, the other thing to decide on is how much cash your experienced runners start with. That can be as unbalancing as too much karma.