I dove many dives (20 or so) in Lake Travis last winter with a 5mm farmer john pants and a 7 mm long sleeved shorty over that (plus hoods, booties, and gloves). I primed the suit with warm water from the microwave before entering the water, and I could last about 50 minutes in 56 degree water. After that (diving doubles), I got really cold. Too cold for safety (I had spongy knees and a pronounced tingling in my calf muscles after one extra long dive of about 75 minutes).
I recently purchased a BARE drysuit, complete with undergarments, boots, hood, and gloves for about $900, and I plan on using that from about November to June, but I haven't had a chance to try it yet (probably this weekend).
However, without a drysuit, I would probably do it another season wet, and winter diving is much better in Lake Travis than summer diving (imho), ignoring the "above water" part. I personally would keep it to 50 minutes or less with my wetsuit, but I have very little body fat, and a full 7mm wetsuit could possibly have made the difference for me.