They are not like a drug which never truly resolves anything or a vitamin which is imperative for your balance and functioning. Flower essences are meant to be taken for a time and then you're off, or you're onto something else.
How long you need to be on one is a very individual factor though. You can change a mood in a minute, but deeply ingrained patterns and feelings from childhood - we're talking months there. Change can happen quickly, but transformation takes time. It also depends on how sensitive you are to essences. Do you feel a shift right away, or is it a couple weeks before you begin to notice a change - maybe?
If you seem to have a problem going off of an essence (symptoms return, and only disappear when you're on it) then it's possible the roots haven't been addressed and it's symptom management only. It also could be that you weren't on it long enough to sustain the change as in the childhood issues mentioned in the above paragraph.
Symptom management is not necessarily a bad thing. I will purposely go that direction if I have somebody coming unhinged and I don't think they are in a good place to go after root issues. I'll formulate something to help them do their day to day until they have their breath back and then we can go deeper.
Examples of day to day blends are things like M&M, Focus, Peaceful Sleep and Ignite. They do contain essences that do some deep duty, but overall they are meant to help you do what you need to do in a day.
Some things that are more root related are Fragment Finder, Heart Healer, and Confidence. If you are serious about going deep, check out the Broken to Whole Kit or look at doing a series of Custom Combos.
You also might be on essences indefinitely because you're attacking one issue after another. Go you! This is what I like to see. I'm not super keen in seeing someone staying on the same thing forever. I want to see you make progress, even if we're doing small variations monthly.
If you are wanting to do some deep work, use a multi-pronged approach. Combine flower essences with another inner healing modality.
And if you're getting frustrated with the enormity of the process of healing, take a look at this blog post my friend and co-author Matt wrote.