VO2 max can be preserved with a third of your previous volume.
VO2 max is the typical measure used in research for quantifying aerobic endurance - it is the maximal rate of oxygen consumption attainable during exercise. I've going to cite two studies, though there are plenty more, that have found that as much as a two thirds reduction in training volume preserves VO2 max adaptations.
The first study, "Reduced training frequencies and maintenance of increased aerobic power"1, put subjects through a 6 days/week training protocol for ten weeks, then split groups into four day and two day groups for an additional fifteen weeks, assessing VO2 max at 0 weeks, 10 weeks, and five week intervals for the 15 weeks of maintenance. Quoting from their abstract, they found:
VO2max in the 4 d/wk and 2 d/wk groups remained essentially the same as the trained levels when determined at 5-wk intervals of reduced training. These results provide evidence that more exercise is required to increase VO2max, than that required to maintain it at the trained level in young adults performing high-intensity exercise. Furthermore, it is possible to maintain the increased VO2max for at least 15 wk by training at high intensity for 2 d/wk or 4 d/wk.
So the group that went from running 6 days a week to 2 days a week was able to preserve VO2 max for 15 weeks, running only one third the previous total volume at the same intensity.
The next study gives it away in their title, "Reduced training volume and intensity maintain aerobic capacity but not performance in distance runners"2:
Ten well-conditioned males (VO2max = 63.4 +/- 1.3 ml.kg-1 x min-1) underwent 4 wks of base training (BT) at their accustomed training distance (71.8 +/- 3.6 km.wk-1) and intensity (76% of total distance > 70% VO2max). Training volume (-66%), frequency (-50%), and intensity (all running < 70% VO2max) were then decreased for a 4 wk reduced training period (RT).
This group also cut volume by two thirds, and even trained at reduced intensity, and found:
Treadmill VO2max was unchanged with RT (p > 0.05) as were resting plasma volume, estimated from haemoglobin and haematocrit levels, and resting heart rate (HR). Submaximal treadmill exercise VO2 (l.min-1), ventilation and HR were also unchanged.
However, they did find a slight decline in 5k performance:
Five km race completion time significantly increased from 16.6 +/- 0.3 mins at week 4 of BT to 16.8 +/- 0.3 mins (12 seconds) at week 4 of RT. Nine of the 10 subjects were slower after RT. It is concluded that aerobic capacity was maintained in these runners, despite the combined reduction in training volume and intensity. However, it appears that training intensity during RT is important for the maintenance of 5 km running performance.
If you want to stay in race shape, you have to keep your intensity up, but you can maintain the aerobic adaptations you've gained with a two thirds reduction on total volume.
A note about terminology, "volume" here just refers to your total distance, "intensity" refers to your average pace over that distance, and "frequency" refers to the number of sessions per week.
1 Hickson RC, Rosenkoetter MA. Reduced training frequencies and maintenance of increased aerobic power. Med Sci Sports Exerc. 1981;13(1):13-6. PMID: 7219129.
2 McConell GK, Costill DL, Widrick JJ, Hickey MS, Tanaka H, Gastin PB. Reduced training volume and intensity maintain aerobic capacity but not performance in distance runners. Int J Sports Med. 1993 Jan;14(1):33-7. doi: 10.1055/s-2007-1021142. PMID: 8440543.
Further reading, featuring similar findings: