There is no peer reviewed data on this, because the question is far too niche and the required period of observation far too long for that to be practical. No-one is running a study where they take a statistically significant group (probably 1000+) of 40 year olds, issue then different exercise prescriptions that they are expected to follow for the rest of their lives, and then assess their rates of cardiovascular events over that period.

The only thing we really have to go on are the World Health Organization physical activity guidelines, which recommend at least 150 minutes of moderate intensity (zone 2) cardiovascular exercise per week or at least 75 minutes of vigorous intensity (zone 3), plus resistance training of all major muscle groups at least twice per week.

Given that a restriction of 60 minutes exercise time per week does not allow the cardiovascular exercise recommendations to be met even when doing entirely vigorous intensity exercise, this completely eliminates moderate intensity (zone 2) exercise as a practical option. Note that there is not adequate data on whether shorter durations of exercise at higher intensity (HIIT and other forms of zone 4-5 training) are an adequate substitute for the recommendations, so it isn't clear whether using this limited time for HIIT would be better than zone 3 cardio.

As for when lower intensity cardio would become worthwhile for health promotion, given that the recommendations are 75-150 minutes of vigorous intensity or 150-300 minutes of moderate intensity, one would need to be engaging in well over 75 minutes of cardio per week before any moderate intensity exercise would be justified, and preferably over 150 minutes before moderate intensity could make up the majority or entirety of your cardiovascular exercise. This, however, doesn't take into account individual preferences, and someone who finds that they are unable to perform vigorous intensity exercise for any reason would certainly be better off performing 60 minutes of moderate intensity cardiovascular exercise than they would performing no exercise.

There is no peer reviewed data on this, because the question is far too niche and the required period of observation far too long for that to be practical. No-one is running a study where they take a statistically significant group (probably 1000+) of 40 year olds, issue them different exercise prescriptions that they are expected to follow for the rest of their lives, and then assess their rates of cardiovascular events over that period.

The only thing we really have to go on are the World Health Organization physical activity guidelines, which recommend at least 150 minutes of moderate intensity (zone 2) cardiovascular exercise per week or at least 75 minutes of vigorous intensity (zone 3), plus resistance training of all major muscle groups at least twice per week.

Given that a restriction of 60 minutes exercise time per week does not allow the cardiovascular exercise recommendations to be met even when doing entirely vigorous intensity exercise, this completely eliminates moderate intensity (zone 2) exercise as a practical option. Note that there is not adequate data on whether shorter durations of exercise at higher intensity (HIIT and other forms of zone 4-5 training) are an adequate substitute for the recommendations, so it isn't clear whether using this limited time for HIIT would be better than zone 3 cardio.

As for when lower intensity cardio would become worthwhile for health promotion, given that the recommendations are 75-150 minutes of vigorous intensity or 150-300 minutes of moderate intensity, one would need to be engaging in well over 75 minutes of cardio per week before any moderate intensity exercise would be justified, and preferably over 150 minutes before moderate intensity could make up the majority or entirety of your cardiovascular exercise. This, however, doesn't take into account individual preferences, and someone who finds that they are unable to perform vigorous intensity exercise for any reason would certainly be better off performing 60 minutes of moderate intensity cardiovascular exercise than they would performing no exercise.