It really depends on the cruise how quickly those sell out. For our Norway cruise we had booked an 11B stateroom and had a sideways cabin assigned. However, later the 11C category opened up and was cheaper, so we downgraded to save money. 11C was in GTY status, so we were not able to book a specific stateroom. Once we got our stateroom assigned, we got an 11B sideways cabin. But a different one from the one we had originally booked.
I think on the more unique cruises on the Magic and Wonder (especially Europe and also the longer repositioning cruises) the sideways sell out quickly. Those cruises attract a lot of repeat cruises who know about these staterooms. But on other cruises, like the 4-night cruises from Miami, chances for getting a sideways cabin are far greater.
And as our experience shows: It is always possible that a sideways cabin becomes open again, even if they were all booked early on. Especially around PIF date.