It's school-dependent. Some schools send out specific IS/OOS batches (Marquette) where if you don't get an interview when they send it out, that's pretty much it. Some schools will send weekly/biweekly invites, some schools will send invites monthly, some schools interview super early and are done with all interviews by December 15th (creighton), some schools interview from September to April (NYU). So it really just depends on the schools.
When the first round of acceptances comes out, you probably won't hear from schools until mid-January as that is when we are supposed to submit our deposit or decline our seat. Once a school has a general idea of their class size after the first round, they can begin to send out more acceptances. I'm not certain if you decline a seat right away if they can immediately fill it with another spot or if they have to wait the full 30 days for first found. Also, keep in mind how schools fill their classes is also different. Some schools will try to fill their class right away, some will only fill it partially and save some seats for post-December interviews, and some do a hodge podge of other things. Again, depends on the school and how their admissions teams like to fill their seats. But a school cannot admit more students than there are seats in the class, so for example, UMN has a class size of 105 so on Dec. 15th they could admit 105 students but no more. However, they traditionally only fill their class partially and save seats for post-December interviewees.
Regardless, I wouldn't think too hard on how schools send interviews. Unless the school gives specific dates on when to expect an interview/when they're being conducted (like Marquette), I would just try to focus on practicing for interviews if you can and enjoying the free time you have now. Find a fun hobby, spend time with friends/family, and do something fun!
