I have been using starters for quite some time and have never had a problem with fermentation, but I am getting ready to brew my first truly "big beer". The O.G. should come in around 22 plato. I want to get the pitching rate correct. Beersmith says I should have 360 billion cells. I know from listening to the show that a starter left for 24 hours is not propagating any new yeast cells. It just wakes them up and gets them healthy. I use a stir plate and oxygenate my starters.
My question is, how long should I let the starter go to get to the right amount of cells?

It depends on the size of your starter and how many cells you are pitching the starter with...

I am making a 2 liter starter with one Wyeast Smack Pack. Beersmith is estimating the viability at 80 billion cells.

I usually make my starters the night before I brew around 9PM and pitch at around 4PM..so what is that 19 hours.....works well for me..I also use a stir plate

I make my starter the night before pitching. However if you are worried about a big beer, why not make a start a few days before, then cold crash it and make a starter again- the night before. That should give you plenty of yeast.

I think this is more a function of starter size. One vial in a small starter will not result in much growth, but 1 vial in 2L starter with stirplate will get you over 2X growth. Obviously growth is a function of time, but all of the growth should be accomplished in 24 hours.

Well, that's helpful! Thanks! So just do a 2L starter on a stir plate for 24 hours. I think in a five gallon batch I will want to let the yeast drop out and pour off some of that wort, right?

At a 1.092 OG (22 plato) you are asking the yeast to do a lot of work for you. If it were me, I would probably brew a smaller gravity (1.050 or under) beer first (with a sufficient starter of course) then use the yeast from the primary to kick start your big brew. Kind of like getting two beers for one (more or less).

If you do a 2L starter, you will want to chill it down for at least 24 hrs (depending on the yeast's ability to drop out), then decant the spent, oxidized wort, pitch your yeast and aerate.