Unfortunately they are fully rng with a percent chance to progress.
I think each crop has 4 stages.
The growth times are expected, not maximum.
Lets say 4 stages and each stage ticks every minute.
If you want crops to expect to grow in 8 minutes, you make them have 50% chance to advance on each tick.
Some could grown in 4 minutes, most will grow in 8 minutes, potentially some could never grow.
There is a name for the curve, but I can’t remember it (or be bothered to look it up!)