Anecdotal knowledge says nothing for 2-3 weeks other than plant growth might get crummy. Too much longer and things start to crash in my tank (lack of something stops growth and algae starts up).
I have lots of fish in mine, never had much of an issue w/ the fish either.
My biggest issues are when a filter clogs or a large fish dies and I don't realize it (3" pleco in the back). Then I've got a big nutrient dump.
Keep an eye on the algae and the plants, if they do something different, do a WC.
What are the repercussions of overdosing ferts (NPK and micro) in a plant only tank? For instance, if one followed standard EI dosing, but skipped the 50% water change for 2 or 3 weeks, would there be negative effects on the plants? This is also assuming the tank has adequate CO2 and high lighting.