My fennel is doing very well, so I'm starting to wonder when I should harvest it.
The bulb should measure at least 5 cm (1.96 inch), but not more than 7 cm (2.75 inch). When it becomes too big you'll risk it to become stringy and tough. Information taken from here.
I have never grown Florence fennel (the bulbing type) or eaten it as far as I can recall, but there are some general horticultural principles we can apply depending on the final use of the plant.
We as gardeners know that annuals/biennials tend to follow a pattern of vegetative growth followed by reproductive behaviour or going to seed. Also that the seed tends to concentrate the essential oils that are valuable for pressing and infusions.
So the first question is what is the final use for this plant? If as a vegetable then we watch the plant to see when it starts to produce flower heads and stop its growth at that point. We maximize the vegetative development and prevent any reproductive growth. Another way to achieve the same result is to watch the size of the bulb and catch it at the moment it starts to get smaller as resources are sent to the flowering parts. We can also anticipate that the fibre content of the plant will increase at that stage to support the top-heaviness to come; this is good for keeping the plant upright but not so good for digestive purposes.
If full maturity is wanted, then allow to grow as long as possible to allow seeds to mature fully, bearing in mind the frosts and freezes to come.