In my experience, regular mail from any state can take anywhere from 3 to 5 days. However, if there are weather issues or just a high volume of mail, it can take a week or two. Here I'll share some information on what to expect when mailing an item, and how to get it to its destination faster if you wish to pay a little extra.
When you put a stamp on a letter and put it in the mail, the process to get it from you to somewhere else starts when the mail person picks it up. Generally, you get your mail picked up every day except Sunday. These are also delivery days. However, a business may not get mail until a business day occurs since they may not be open when their mail comes in on a Saturday.
From CA to IN you'll have to give it 2 to 7 days for standard mail. However, if you pay for priority then you'll probably be looking at 2 - 3 days. If you're going to want to get it there with a guarantee that it will take 1 to 2 days, then Priority Mail Express is going to be your best bet.
If it must be there next day, then I suggest using UPS and their Next Day Air service. While it costs a lot, it generally will be on time as long as there isn't a weather related emergency. They also have a Next Day Air Early AM service that is very expensive, but useful if there's a rush and you don't mind paying the extra.
1 - http://www.fewtek.com/california.htm
2 - https://www.usps.com/ship/priority-mail-express.htm