The Gardening events lately, especially with the addition of hard tasks, have been designed with the idea in mind that people will share with you. If you don't miss a cycle, the average player will get just enough butterflies to finish the normal tasks. But if you're unlucky with spawns or catch rates, as I have been in the past, you may not finish at all. It's expected of you to share and have others share with you, so get a lot of friends and hope they have the kindness to share with you and not give you bugs you don't need.
There is someone on my friends list who somehow managed to finish the entire event within a day every time. Not sure how many friends they have with amazing luck to give them all their bugs, but somehow they do.