Just 30 mins a day?

[left coaster]

10K's and under with lots of time to warm up and cool down. You could probably do most of the Pete Plan with 45 minutes a day... I'm not sure you would really make progress with 3 days but it would probably be enough to maintain if you went hard each session.

I generally don't do more than 45 minutes on the erg, just get too bored. If I'm training 2 on, one off, I generally make good and steady gains. Working myself back from a too-long break currently and doing the 2 on 1 off thing. So long as I'm working hard I know I'll make progress -- I spent too much time over thinking my training in the early days.

Many endurance athletes that i know make fine progress with 3 times a week. Not World class or something but still ok. So Why not?
When i was a runner many years ago i train 3-4 times a week and still did HM around 1:30

Everyone is different... with only 45 minutes 3 times a week I know my progress would be slow, if at all. I've actually tried it and didn't progress. I could maintain but that 4th session you mention I think is key. With a 3-4 rotation you're basically one day on, one day off, without the 72 hour gap that happens with just 3 sessions. Also, as a marathon runner, I expect your sessions were considerably longer than 45 minutes.

[gooseflight]

What if one only have 45 min 3 days a week?

With 3 x 45' I'd do something like two low-rate 45' UT2/1 ish pieces and one truly hard interval session, maybe rotating 8x500m and 4x1000m on a weekly basis.

^This