Once you notice dust building up on the intake areas of your case, simply clean it off. Be sure your computer is turned off before cleaning, though. You don't want to have what you're dusting off getting sucked inside the case. I use a medium-sized paint brush to lightly dust off the outside and inside of my case. I also do this outside of my house and on a table. Carefully hold your fans from spinning when dusting them.
I wouldn't worry too much about a little dust unless your home is very dusty. If it is, then you may need to clean more often.