i only sat up on hills when i was below 12 mph. At IM lou this was plenty for me.
Also, I noticed that it is easier for me to stay aero on the road versus sitting on the computrainer.
Way easier on the road. On the road the ONLY place that is comfortable for me is down in the aerobars. On the computrainer I can barely handle 10-15 mins at a time - I usually alternate going up and down for a whole trainer ride.
As others say, I too think, the longer the better. One thing to do, though, is make judicious choices of when you get out of aero - on hills, in a more technical section, with the wind at your back. At IMAZ last year, I took a minute or so just before the turnaround to stretch on the bike, get out of the saddle, and get ready for the ride back into town, when I didn’t get out of aero at all (imaz is 3 loops of out& back; the wind at your back on the out, against you on the back).
good luck,
tom