If an adult has been taking ballet classes for some time and working their feet well in class, the required strength in the forefoot will not take long to develop. However, for the complete newcomer to ballet, learning how to isolate certain muscles in the feet may take some time. Our bodies develop ‘motor patterns’ of movements that we do regularly, and it is important for this isolated foot control to become second nature to the dancer before commencing pointe work. There are so many other things to think about while en pointe, that the dancer must be able to easily control the position of the toes in the shoe to have optimum control and therefore safety en pointe.
The coordination required to control the feet en pointe is something that is developed over years of dancing, and, as for any student, I would expect an adult to be dancing at least 3 classes a week for a year, and ideally regular classes for several years before considering pointe work.
All the points discussed in "The Perfect Pointe Book" should be addressed for anyone of any age (men included!) before progressing onto pointe. It gives guidelines and tests for range and strength that are so important to prevent injury. It is not impossible for an adult ballet dancer to progress onto pointe, but it will usually require a lot of work and dedication to achieve this safely.