>Do I should rewrite the source code?

Yes, most of the people will suggest to rewrite. If you want to benefit of OOP environnement, making all the adaptations from your existing code will cost twice the amount taking twice the time as if you are rewriting.

>How long I can change all application?

This depend on the size of the application. Programming in OOP is faster than what is used to be in 2.5 but it takes a learning curve.

>What should I do?

I would suggest to start with a framework and a good analysis.