That depends on the tightness of the binder, how long you should wear it. If it's more of a lycra vest type and not especially restrictive then you can wear it pretty much 24/7 if you want to (I have one which I wear almost constantly post-surgery for the next few weeks), if it's high compression then you are going to want to keep an eye on how it feels and how freely you can move in it. Generally they recommend not sleeping in those, and only wearing them for a part of the day. By the way I would recommend using a combination of something like a sports bra and a lower compression binder than one that is ridiculously tight as the latter can cause everything from breathing difficulty to digestive issues. You should not be wearing anything that is physically painful to wear. It should feel like a regular compression vest, not a torture device.
Can't answer how it's 'supposed to feel' because you are meant to find the binder that feels right for you, other than that you are looking for reduced chest movement and something that hopefully flattens the area more than without - it will not completely flatten it though unless you have an extremely small cup size to begin with. I wouldn't advise trying to flatten the area so much you start wearing something that makes your ribs hurt. Pain = not doing it properly.
Side effects, there aren't really any of these unless you bind for years on end or use ones that are too tight. If using for years on end all the time you'll end up altering the elasticity of breast tissue and skin (esp. the Cooper's ligaments) which result in saggier boobs. But they are going to sag with time anyway, technically.
Frankly using the right compression level binder it feels good - the material is usually something synthetic like lycra that allows clothes over it to move freely, and with the right compression it can feel just as comfortable as not wearing and feels supportive on the whole torso if using a long binder. It obviously reduces the amount of gravity acting on the chest when doing physical activities as well.
Vest-type binders which stretch a bit with use and washing meaning they are uncomfortable first time on and then after a few uses become more wearable. They can be difficult to get into first time if not using one with a front zip. Front zip and clip types are easier but are usually thicker and restrict more movement (medical grade binders are like this as well) but are more expensive so I would recommend buying a cheaper one first to try it out. Getting the right size is trial and error at first to be honest, unless you have access to several to try.
