when is it faster to walk back to base instead of recalling?

i was always told that once you on the steps on the outer edge it is faster to walk

Depends on your movespeed, or on how annoying your teams trundle is at the time.

Depends on your MS but if we guess you have 350 MS then the distance needs to be greater than 2800 for recall to be faster. Basically if you look like you're a decent bit within Lux ult range you can walk.

I typically just walk back to the fountain if I'm anywhere within my base.

Just do a custom game and find out. It's different depending on your movement speed. So test it with different MS items you would get throughout the game.