To make your life easier, I'd suggest that you create a global variable to manage the real-time issue while you're still working on the game. This is very important to ensure you don't waste any time while developing but still use the exact same system that the player will ulimately use.
Assume this variable is called called HarvestMultiplier. So, set your plants to be whatever harvest length you want, and then create a single event Plants(family) On creation > Set HarvestLength to Plants.HarvestLength*HarvestMultiplier. If your multiplier is set to 1, it'll be real-time. If it's set to 0.1, it'll be ten times faster than real-time. 0.01: 100 times faster, and so on.
If you want, you can also come back to this later to adjust overall game speed from a single variable without having to manually change every plant, which will probably be a huge chore later down the line.