In general, hair has to be about ¼" long for wax to be able to grab onto it. If it's not long enough, then you (or your technician) may not be able to remove it, especially if it's thick. Does that mean you should wait until it's really long for your next wax? Not at all. In fact, it can make getting a good wax way harder. If hair is 1/2" long or longer, you can run into some real problems.
When you try to wax long hair, there's more of a chance long hair will break either below or above the skin's surface. Both of these are bad outcomes; either the hair will grow back faster, or you risk the chance of getting ingrown hairs. It's also probable that not all the hair will be removed, because when the wax is applied, it pushes hair down flat against your skin. If some of the long hair covers some hair underneath, it won't be coated in enough wax to be removed. It also just hurts a little bit more the longer and thicker it is.
You can avoid some of these problems by simply taking off some length. Naturally, you don't want to make it too short. The best thing to do is use hair clippers with a guard attachment that will quickly trim it to an even ¼" long before waxing. If you're going to get a professional waxing job, though, your waxer will be able to trim the hair down for you. Whatever you do, don't use scissors—you run the risk of cutting some hair too short.
If your technician doesn't trim down long hairs, they're probably not the right technician for you. License and skill don't always go hand in hand. A good technician will be fast (so there's less pain), hold skin taut so there's less pulling, and use the right amount of wax and type of wax for your particular hair type and skin. Waxing is painful, but there are steps that can be taken to make you more comfortable, and your technician should be taking them without prompting.
If you're waxing at home, be realistic about the fact that you're not a professional and there are limitations to what you can and should do, when it comes to waxing. There are places you shouldn't wax on your own, and if you don't want to go to a professional, you can shave it. Be aware that the steps in place for waxing are there for a reason. Not properly prepping your skin, applying too much wax, or removing hair the wrong way will cause irritation, bruising, and breakage.