Swimming how much off season?

Okay tri ladies- I am wondering how much swimming one needs to do in the off season to stay somewhat fit? I say off season, because I live by the lake and have so far only swam in the lake. However I am getting a bit more serious (well I just do not want to be the only bike left in the transition anymore) How often should I be swimming just to keep up fitness- bare minumum until I can get back in the lake? I am going to go to the local pool for the winter about six months?? I really just do tris for fun, but have to start from scratch every April with my swimming, if you can call it swimming, but it is getting better. I just ordered the book Triathlon Swimming Made Easy. Oh maybe it says in that!

I generally like to make a point of swimming a varied workout (with some drills, intervals, sometimes just straight yardage) at least once a week, during the off season. I don't really put in that much time (1/2 hr), but it sure beats starting over. This year, I was unable to meet that goal and was hurting pretty badly, come April. With hitting the pool now twice a week (with incresing yardage and a couple of Oly tris later), I think I'm ready for my 1/2 IM. Think.

Angie