There is no magic figure, but there are clear signs: when your profit grows, when you hire people or when you want to limit your personal liability.
The tax difference
A company pays corporate tax at a fixed rate, while a freelancer pays income tax on a rising scale. From a certain level of profit the difference is noticeable.

Advantages and drawbacks
| Freelancer | Company | |
|---|---|---|
| Set-up cost | Almost none | Notary, register and capital |
| Liability | Unlimited | Limited to the capital |
| Taxes | Income tax, rising scale | Corporate tax, fixed rate |
| Paperwork | Simple | Full accounts and annual accounts |
Changing too early costs money; changing too late costs more.
Numbers first
Before deciding, ask for a study of your case: it also depends on how much you want to take out of the business and on your plans for the coming years.