Should I just exercise flat out and try to sustain that as long as I can, or is there a level of vigorous exertion below flat out that leads to better results because of the longer duration?
Do you stop afterwards and catch your breath? It was vigorous. Bonus if you broke a sweat.
There's no need to be more qualitative than that for most folk (a "suboptimal" schedule consistently applied being vastly preferable to an "optimal" one inconsistently), but if you absolutely insist on getting quantitative, see: https://pacompendium.com
[in particular, see https://pacompendium.com/sexual-activity/ : a light pash might be twice as much activity as just sitting on the couch, while "scaring the horses" can be six times]
If you can make yourself do proper HIIT regularly you will become batman but the mental barrier of intentionally making something "suck" as much as possible (and then some) is really tricky to get over. Even once you are "on the wagon" it's a difficult habit to maintain.
I find 30-60 minutes of moderate daily cardio to be much more approachable on a regular basis. It also feels more meditative to me than rushing through a 20 minute hellscape. I can sit down and write code for hours after getting off the rowing machine. After a CrossFit routine I just want to nap all day.
> When we say ‘vigorous’ we mean activities that make you sweat, increase your heart rate or increase your breathing. Please think about vigorous activities that you may have done at home or at places of work other than your home, as well as vigorous recreational activities or conditioning exercises. Please think over the last year and indicate how often you participate in vigorous activities.