likelihood of Royal Flush occuring

Hi.


I have a question. I hope I get the essence across without confusing everybody...


Can you calculate a "safe period" of someone getting dealt a Royal Flush pat (in 5CD)? Say you wanted to be 90% sure you get a royal flush pat in your poker career, how many hands would you have to play?


I do not know if there is an answer or if it is a ridiculously stupid question, but please humor me with your insights.


Stephan