It depends on the IOP, the only way to find out if you're a candidate for a specific program is to ask them. Ive been in an IOP and will probably return soon. The one I go to is geared towards adolescents with mental illness but young adults and those struggling with non mental illness related things can go too. There's a psychiatrist there you can see daily which has helped me a lot. The groups also have drastically changed how I think about things as well.
This IOP is 3-5 days a week Monday through Friday and also offers PHP. If you're PHP its 12-3 and IOP is 3-6 but some people stay all day from 12-6 if they need to. There's no drug testing and you can miss days as long as you still come at least 3 days a week.
I strongly suggest the program I'm in to anyone that can go because its helped a lot just having a community irl that I can talk to and can monitor me especially during times like now where I'm not stable.