Basically, its this....
Tin shovel
One bean combo 0 - 40
Two bean combo 41 - 80 =
8 days
Bronze shovel
Three bean combo 0 - 80
Four bean combo 81 - 160 =
16 days
Silver shovel
Five bean combo 0 - 160
Six bean combo 161 - 320 =
32 days
Gold Shovel
Seven bean combo 0 - 320 =
another 32 days
Eight bean combo 321 - 639 - after
one more day planting you get your final laff boost but you can continue to max it just to get a fanfare and confetti!!!