What is the maximum time-span of a referee? I applied for a PhD program in 2017 and got rejected. I'm preparing to apply again in 2018.

Could a professor who gave me a recommendation letter last year refuse to do so again? If so, why, and how could I approach this issue? Is there any unwritten rule regarding how long a referee will provide a reference for a student?
Maybe there are cultural differences, but when I am writing a letter of reference for a student, it will hold lifelong. The rationale is simple:


It has a date on it, so the other side can see when it was issued and decide by themselfes how valid my interpretation of the student's performance is at the current date
I'm writing my true opinion on the student, there is no reason to change my mind. 
If someone want's an updated version, they can call / contact me via e-mail. 


I know of no official or inofficial rules (in Germany).

The situation might be different if you need a new reference letter. Then I would only write it if I can tell something new about you. 

So if you need an updated letter of reference, the referee should recieve some content from your side to put into this letter, together with a brief explanation of your goals, how you improved, etc.