When to buy motorcycle luggage is a good question which probably depends on the biker you ask and their personality. Me I procrastinate so I would probably invest in some form of motorcycle luggage real close to the first long trip I plan to take.
Ok honestly the last time I was already on the road and stopped and dumped the backpack for a tank bag that fit all my stuff. Proably not the wisest move but hey it works for me! Anyways when you want to buy something to store some things on the bike on you have lots of choices as to how you want to go about doing it. You can either go with some sort of tank bag or saddlebags and depending on the bike you might or might not have a choice. For example my chopper has to have a tank bag due to the way the lights on the rear are setup.
So with saddlebags you have that nice nostalgic look or a more modern hard case look for sport touring bikes.
So really its up to you but I like the versatility of tank bags or a some throw over classic looking leather motorcycle saddlebags.
So I geuss their is no definite answer as to when is the right time to buy motorcycle luggage. I would say the smart money is when you can catch some on sale. I have had mine for 9 years now and its still perfect.