When to have your outdoor grill degreased depends on several factors.  How often the grill is used, obviously the more the grill is used the faster the grease will build up.  The way food is cooked on the grill, grilling on high heat allows the grill to burn off a lot of the grease and leave a dry ash inside the grill.  Barbecuing foods on medium to low heat allows the grease to collect inside the grill instead of being burnt off.  If the grill is used regularly and cooking is done on medium to low heat, the more often the grill should be degreased.

How difficult  will a grill be to degrease?  This depends largely on the type of deposits that are inside the grill.  Brown greasy deposits are easier to clean than the heavier carbon deposits.  When grease is continually heated, eventually everything is burned off except for the carbon.  Carbon is the element on this earth that bonds the easiest with everything.  To determine how difficult it will be to degrease your grill, take your finger and rub it on several different spots on the grill.  If the brown greasy deposits are not to heavy, you should be able to rub them off with your finger and see the stainless steel below.

If there are black carbon deposits, take you finger nail and scrape it across the carbon deposit, if you can scratch off the carbon and see the stainless steel below, then most of that carbon deposit should probably come off.  If you scratch repeatedly with your finger nail and you can not see the stainless steel below, then you have very heavy carbon deposits, some of this will come off, but probably not all of it.

Obviously if you grill is new, or not more than a few years old, it will be easier to degrease before the carbon deposits start to develop, and easier to keep clean if the degreasing is done on a regular basis.