What should be the maximum validity recommended for extending factory calendar

Hey All;

Can you please throw some light on my doubt that Maximum which period we could extend the factory calendar in SAP?

I am asking this question, since our client has asked me to extend factory calendars of too many plant till 2099 year.

Is it recommendable ? SInce i have replied client that it is not recommendable , so can anyone guide me whether it is recommendable or not ? If not then why ?