Asking reference letter earlier or later if I need to go mandatory service for my country

I would suggest explaining your situation to the professor and asking him to write the letter now.

Later on, if you do manage to do some extra work you can ask him to revise the reference letter. The revisions will be quick and easy, but writing a reference letter from scratch after two years might be difficult.

The professor should not be disappointed if he writes the letter but you decide not to continue in academia. A good reference letter will also be very useful when applying for other jobs.

Agreed! In the past I have asked faculty to write a recommendation letter because I got to know them during my class, but wasn't planning to attend grad school for a few years. They may be willing to keep a draft on file that can be printed out in two years or give you a number of sealed copies that have generic salutations (Dear Reviewers). Many professors would not mind doing this and actually would prefer to write a letter that they can write with ease because their memory is fresh, rather than the difficult task of trying to remember the student's performance years later. The letter will also be of higher quality if written soon!

Good luck!