Time for open water swim.

Hello, I am a new member, and a brand new triathlete(in training).

I was wondering, in general how much time would you add for an open water swim compared to a pool swim? I would imagine it would depend a lot on location (ocean/lake) and conditions. But in general what do you think is a good rule of thumb?

I have always enjoyed running and biking. I have tried to train for marathons, and got burned out with just running, and I never had a good group to train with or keep me motivated. I just started swimming this month (I could use some tips on form also). I really enjoy the transition and what each element of the triathalon brings to the table. It keeps me excited and motivated, and it doesn't get boring. I have been doing drills in the pool, but today at the end of my session I decided to do a 400(the swim distance of my first triathalon) nice and easy. I finished it in 12:00.

Thanks for your advice!