you could probably get a year... a year and a half. Small Pbass grow about an inch a month. Faster/slower depending how much and what you feed them....
Yes, you could possibly keep 3 occelaris in a 180 for their entire life. It would be a little tight when they hit the 18 - 24" mark, but I've seen it done. Obviously a bigger tank would be better, but I still believe it could be done.