How many values in an "in" clause is too many in a SQL query?

I have a SQL query that uses the values of an array in its WHERE clause:

 $ids = array 
         ( 
           [0] => 1 
           [1] => 2 
           [2] => 5 
         ) 

 $ids = join(',',$ids);   
 $sql = "SELECT * FROM cats WHERE id IN ($ids)"; 

My question is how many ids are too many?
Will it effect the speed?

Thanks all