It has nothing to do with the amount harvested. Rank ups only care about # of harvests. It takes 512 harvests total to reach rank7, so you can math out how many days it takes if you harvest once per day... >_>;; That is why people use Revival Roots (1hr) or Arborescent Seeds (2hr) to speed up the process, but that method is very annoying as it requires a lot of time spent just zoning in to Mog Garden every 1-2hrs.
This is why I asked in the QOL thread for a new fertilizer that works on all nodes and converts all drops to crystals.. 512 harvests from furrows alone is a mountain of absolute trash drops, and I'd much prefer a mountain of crystals which I can actually use instead of inventory clogging junk that sells for 10g each.