If I did 8 hours a week i might get down to 24:28... :)
I think the 25m pool was worth about 3min over 1500m for me. I got to 16:25 for 1k in the 25m but couldn't break 18 in a 50. This was after what felt like 8 hours a week for years but was probably more like 2 hours a week for 6 months.
From a competition standpoint from memory the conversion is just short of 30 seconds.
For an elite swimmer? Probably more for tri-hacks.