Hello, daily stretching and physiotherapy is very important to prevent further development of ballerina.
I recommend you to use a night splint to keep the feet in a 90 degree position. There has to be permanent stretching during hours.
Please note, that ballerina (pes equinus) isn't an easy thing, especially when there is a big amount of ballerina.
After lengthening, I had 5-6 cm of ballerina and needed approximately 1 year to recover from it completly (one feet was faster, about 8-9 month), and I did 1-2 hour of stretching/day during 6-8 month.
Greetings Morpheus