wait, so the higher GROWDUR is, the longer it takes to farm, but also, it increases exp gain?
No. EXP gain only comes from planting and harvesting. Growdur only increases the time it takes to harvest.
My little "spoiler" above contains a new plant_standard that makes plump helmets, strawberries, and pig tails food take about 6 months to grow, and the rest taking a little more than 8 months to grow (this is the same ratio as it currently is, just stretching time out)
What this does is makes your fortress require more space to feed the same amount of dwarves, since your farmland is occupied for at least two full seasons on a single harvest, rather than 3 harvests a season. That means you need to do six times more farming than you'd usually do.
This way it takes more than a single farm tile to feed a dwarf. The exact amount of tiles required depends on the skill of your growers. Dwarves require about 8 food a year. This makes at most 2 harvests a year, meaning with legendary growers you only have a surplus of 2, which doesn't provide much room for making drink for the dwarves. So with this, 2-3 tiles PER DWARF and only for food production is a good rule of thumb. This is greatly different from how a 5x5 area could feed and provide drinks for an entire fortress with legendary growers.
Make sure to take enough food to last the growing season, and enough seeds that your first harvest can supply you until your next one.