You have not indicated the status of your service in the central - government. What I mean to say is whether you are in the probation period or have successfully passed the probation - period.
If you are under the probation - period under any Public - undertaking of Central - government or an employee of central - Goverment, the maximum casual - leave to be granted would not exceed beyond twelve days. Supposing that entire Casual - leave is granted to you at a time for the submission of thesis, you have to manage the entire year without C.L - a condition which may appear difficult to manage your affair without having any leave.
The other option is to pass the probation - period satisfactorily and then seek for the study - leave extendible to two years with the permission of the appropriate authority. However, such leaves are treated as Leave Without Pay but you would be enough time to persue and submit Ph.D.