if i had head gaskets changed already , how long should they last?

The life of the engine, as long as you do not overheat the engine.

They should last a long time provided the job was done properly - head checked for warp and torqued correctly.

If its from a aluminum head that had a previous warp and it was resurfaced can last a few years, maybe 3-5 years if not resurfaced only months to a year or so. A pre warped head will always warp again if you run it long enough, now if head was checked for being warped and it did not warp but gasket just blew (which is rare with aluminum when ran hot) then it should last until its ran hot or longer if not ran hot. This is my hard learned experience, now that I know this first hand, if one of mine ever warps it gets a none warped head replacement either new or one I pull from junk yard myself and have tested for warp first then pressure tested but if its warped or cracked I take it back, pull another and I tell the junk yard owner my intentions before hand. Its not a myth its true, But same is not true for cast iron heads but your Saturn is a aluminum head if its the 4cyl.

As too does the v6 it has aluminum heads too.