You know, hands on the hoods, forearms parallel with the ground.
As for me: not very long. At the end of many workouts I’ll cycle through 5 minutes in the drops, 5 minutes on the sticks, 5 minutes aero-on-the-hoods…by the 3rd 5 minutes on the hoods it’s hard to maintain that position.
I can stay in the drops forever, limited by hand position numbness. I can only stay aero on the hoods for about 3-4 minutes at a time, but I’m working that number up. If I can rotate between drops and aero on the hoods, I’m pretty content. I am switching to somewhat narrower bars now, and I’ve generally found I’m more comfortable on narrower bars in all positions. I ran 40s forever; Venge came with 42s, and I’m just now changing them out for 40s again. Cockpit setup can make a difference.
All day……seriously. I rarely get off my brake hoods, and I usually have my forearms close to parallel.
Same for me, most comfortable position personally but I also spend a lot of time on the turbo in my TT position. Also found that angling the hoods in means the I can rest midway on my forearms on the tops whilst I’m down rather than just wrists behind the hoods.
This is my default position, followed by the drops.
I can spend hours in tops in 45-65 degrees forearm position but just minutes in 90 degrees…
Last time I measured it was 3x15min in aero road position about a month ago. February this year was ~4min max. Just do it all the time it’s comes quickly.
Up to 70deg endlessly (at least 15h). 90 Deg about 30 to 60min
I’ve never actually paid attention to it (aero position on the hoods) but I think I could stay in that position as long as I want. I often prefer that position over the drops simply because I don’t really like braking on the drops.
I can spend as long as I want - have for several hours. If you cannot, I think its an issue with your bike fit - not a flexibility issue. I typically side in this position and hit the drops only for big sprints on tucking in on a descent.
I mean to say that your seat to handlebar is prob too steep for your flexibility, and thats why its hard to actually be hunched over that much. Should prob put some spaces or flip stem up / get higher angle stem and you should be able to get into the 90 degree elbow position comfortably.
Only a few minutes at a time for me. I try to do it a lot on z2 rides, and i am getting better. On the other hand, i can ride in the drop forever, so not a flexibility issue, i think its more of a reach issue, maybe a change in stem length would help
Oh yeah, same. I’m on the drops ~100% of the time during a crit/road race.
It does load the upper body in a different way. More load on the forearms and triceps, I consider aerohoods to be arms parallel to the floor. This is what I find takes the training not as you say flexibility. Most people the back is the same height in aerohoods or the drops, but the extra speed comes from having the arms in a better position.
Are you sure we’re talking about the same thing.
Aerohoods being arms flat and parallel to the floor. The wider and safer cousin of invisible aerobars.
Yep, same. Aero pos’n on the hoods means forearms parallel with the floor/ground. Although I do find this fatigues my lats somewhat my experience is similar to what you mention: tricep fatigue. In a weird kind of way…seems like the tricep fatigues right at the elbow.
But the advice on this thread that bike position can have a major impact on the comfort of this position is probably true. I’m not a bike position expert and generally speaking changing my stem length by 20mm to 30mm or moving my saddle around several centimeters doesn’t really bother me. So I’m the last guy in the world to advise or speculate about bike position. 
This is what ‘aero hoods’ position is IMO. Forearms on the bars isn’t on the hoods
Yes I agree! I never steer with my elbows. Don’t trust my bike handling skills enough.
I mean sure, but raising the front basically defeats the purpose. I’d rather keep my back flat and be in the drops than have forearms parallel in a more upright position.
For almost all of my (and probably most people’s) racing, the drops are more appropriate anyway so that’s how my bike is fit.
Sure but if the rider can only be there a few minutes due to discomfort then there’s not much point anyways. The bike fit is too aggressive for their flexibility. The considerable aero gains come from long durations in an aero position, not just popping down there for a few minutes.
If you could raise them up a bit so they can sustain it for longer, it will be a net positive change.
I disagree. The body can adapt, the aerodynamics can’t. As I said further up in Feb, I was doing 2-3minute. Now 3x15minutes is easy. Bike fit hasn’t changed. Flexibility never came into it, but arm strength did.