I like to think of it in terms of blood flow, systemic effects and local tissues:
In a relatively avascular area, such as intra-articular, you should theroretically get a particulate steroid to stick around for a long time. Therefore, I try not to do it more often than q 3 months. I will do it closer together on a rare patient who has no other option while we work on other things - meds, PT, etc., or while waiting for surgery.
In a more vascular area, such as the subacromial space, around tendons, nerves, etc, you may be able to get away with it a little more frequently, but look at the steroid load you are giving them. The more vascular the area is, the more systemic a response you will get, along with it's many side effects, including bone metabolism, GI ulcers, weight gain and risk of avascular necrosis of hips, e.g. Also local steroid effects such as local tissue breakdown and risk of infection.
If you limit any injection to no more than 3 - 4 per year, and use the minimal amount of steroid neccesary (another debatable subject), you should be ok. Go above that and you had better have some literature or a good expert witness to back you up in the case of a serious adverse event.
An even more debatable subject is what to do with the patient in need of multiple injections for multiple locations - shoulder impingment + lumbar stenosis with radiculopathy + knee OA + hip bursitis. There's no real studies on this kind of patient. (And yes, I knoe there's other things you can do - NSAID's, PT, etc. I'm just talking about the patient that's tried them and could benefit from local steroid injections.)
When consenting patients, I tell them I use the minimal amount of steroid with the minimal frequency and minimal # repeat injections to bring their problem under control. If their pain problems cannot be controlled well enough or long enough by that, we need to look at other options.