I've got only one race under my belt, but I found that my pool time was pretty close to my OWS time. I wasn't timing myself very precisely; I just know that I need to get out of the pool by a certain time in order to get to work on time.

I was figuring on a 25-30 minute swim including rests. I took one big rest during the swim and ended up finishing around 25 minutes. It was a small lake, very little wind, so I don't think currents were a big factor.