Collect all rewards for Lumi's garden requires you to beat each level 12 times, each time picking one of the remaining rewards.
That's a total time of 12 x 4 x about 10 minutes and 12 x about 17 minutes, not counting all the times you die on the boss' last wave.
(or as a sum: 12 hours in pure gameplay)
It doesnt matter if you play on normal or hard.
However, you can't unlock any rewards in 'free eden' mode.
Since you will have to redo levels over and over again if you try for the 100% and/or 800k achievements, this one comes 'naturally', given your patience didn't run out until then, or you are simply so god-like that you clear each level with 100% and 800k on both difficulties below 12 tries.
Kudos and my everlasting respect to you, if you manage the latter.